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What seating depth is actually doing...actual node or just pass/fail?

You are on the right track. You don't miss the point about the first mode being lower with a heavy barrel. However, I am pointing out the modes that count more in this load tuning discussion are the ones that cause us to resort to heavy barrel sections when we want accuracy and how those affect tuning.

No background in structural dynamics is required to know you would use a heavy barrel for a test rig or target rifle, but why is that if they lower some modes? Why are heavier sections not worse when they lower certain frequencies and how do they relate to being able to see or not see dramatic effects from load tuning?

The low note you are sensing is the first mode shape of the barrel and or it's support which is also very low frequency compared to other modes. When all that weight is a cantilever on the receiver, the simple view is not deceptive. Add weight on a cantilever, the frequency goes down. The first mode of the barrel also goes down.

You were not wrong, but some frequencies (modes) go up with stiffness from increased section (weight), and they can also move down if the weight isn't distributed correctly. This is why barrel flute cuts may or may not help with accuracy and why some barrel contours become popular in the accuracy crowds and others don't. It is also why your first statements were not wrong, heavier barrels lower some mode shapes, especially the low mode ones. The point being, why do high quality match guns seem to be insensitive to load tune variations?

The barrel first mode is usually low enough that the bullet exits within a fraction of that oscillation, however the second mode has enough time to oscillate many cycles.

Oversimplification in the thread means folks are missing the point of load tuning affects and why they are or are not easy to see in some guns and very obvious in others. I am not good at making this simple to explain or easy to understand, sorry. I'm dyslexic and admit I am a horrible writer....

It takes many words, diagrams, and math, to explain ballistic structural dynamics. I know what happens if there are this many words and then we add equations, especially the kind where you have to jump back and forth from frequency domain to time domain in order to have a good discussion.

Once a presentation starts up with phase diagrams, Eigen values, decibels, and Bode plots, folks start getting bleary eyed and head for dream land. Then I pulled the lanyard and the demonstrations got more interesting.... Who doesn't like to see ordinance plowing into targets or would they rather hear a math class on how a barrel and load were tuned.... Most folks just want to see the targets and I don't blame them.

BTW, there were several folks asking for a physical reason about why or even if the barrel mode shapes are mostly vertical. The reason most sporting and competition bolt guns have a strong vertical pattern, is due to two main reasons, 1) the low mode where the CG of the system is below the centerline during recoil, and, 2) the pressure shock inflation of the barrel against the gravity sag.

This description discounts the situation where your barrel touches a stock. All bets are off when that happens for this discussion.

The gun barrel is already sagging down due to gravity. If you consider the shock of the pressure extending the barrel and exciting the first modes of barrel/stock oscillations, the motion of the barrel inflating due to pressure counter acts the barrel sag and that first reaction becomes mostly vertical. The recoil lug and receiver are not symmetrical in bolt guns because they are underneath but even guns with symmetrical barrel supports move vertical on the shot due to that sag and barrel inflation. However, that said... The low mode shapes are not always just a straight line up and down, but many are nearly oval or ellipse with the long axis vertical. Get your tune exactly wrong (or have sloppy charges), and you will see a horizontal dispersion in many systems.

When weight is cantilevered over the length of the stock fore end, with the back of the stock touching a rear bag for example, there is a very low frequency mode associated with the whole gun and the weight of the barrel and receiver on the stock. The barrel first mode can be on the order of roughly 100 Hz.

A bullet is out of the muzzle before this mode gets even a half cycle. You can't really tune out this mode. Not saying we can ignore this mode, but for this discussion you can't do much about it since it is cast into the system. You can't go slow enough to take advantage of it is a different way of saying it.

The response of the first mode of the barrel on the stock doesn't play a strong role with respect to OBT tuning, but it does play into which effects are important on the whole because it is always there and potentially very large. The recoil response of the system on the whole is a huge effect on the difference between the point of aim and the motion of the muzzle as the bullet exits because the whole system is moving before the bullet exits.

It is not what folks want to hear, but it is why shooting technique matters. Rest or hold the gun wrong, and watch what happens next, regardless of how carefully your ammo is crafted.

The same ammo in a heavy system is more likely to be more accurate than that same ammo in a light system, mainly due to the interaction of the first mode with recoil. By the same token, get the load tune right in a light flimsy system (or more importantly get it wrong), and you can see bigger differences on the target. The potential changes here in the light barrel are larger, but the two baselines probably tip in favor of the heavy section to make it a moot point. You pick the heavier gun and barrel if you want accuracy... but nobody like to hump a heavy gun....

With respect to load development and component selection, those low mode frequencies and their effects are not always load tunable. Think about an AR-15 where the barrel extension sits in the upper, and then the whole upper rattles around on pins. As a mental exercise now imagine taking a pencil barrel profile versus a varmint profile in that same gun, and doing a modal analysis.

You will certainly find lots of the mode shapes shifted left on the PSD plot due to the weight of the heavier barrel, however, now think about what you have observed with different AR rigs between light weight pencil barrels versus heavy barrel varmint rigs and match barrels. No structural dynamics background required to guess which mode shapes matter more. Even when we lower some of the frequencies due to the weight of the barrel section, we are jacking other important ones with respect to load tuning. All while sitting in a sloppy pin interface between the upper and the lower.

With the little pencil barrels, the support frequencies shift up but many of those are a fraction of their oscillation compared to the bullet exit time. Keep in mind that some of the frequencies are very low and the others are in multiples of those and some are even in the kilohertz range. Both the thin light flimsy barrels and the thick heavy stiff ones have low first modes. It is those second modes and higher that are different between them, and you don't need to know much about structural dynamics to know that heavy section bull barrels and light section pencil barrels don't do the same things even if they both have a low first mode.

Some choices we make in the gun system set the tone for what the potential group dispersion will be, and as a result only some things can be load tuned. The reason for confusion is the share of the ones that are affected by barrel exit timing and load tune may be very dead or low amplitude on match guns because the barrels are heavy and stiff. The heavy choice lowers some frequencies, but improves recoil issues and stiffens important modes.

If you could hypothetically make a magic barrel that was infinitely rigid and weighed little or nothing but still had a moment inertia, you would still have a low frequency issue from the receiver, stock, and recoil. The recoil modes would force you to add weight to the system to reduce recoil displacement. But think about the share of the target dispersion that could be affected due to the load tuning. Only certain frequencies are within the reach of the load tuning. A key concept is to remember that amplitude is just as important as frequency when all is said and done. YMMV.
Thank you very much for this. I'm sure many who read this have their head spinning. 😵‍💫 But, it's the kind of thing I really like to dig into. Because I'm aware of your education and extensive experience with analysis of such things, I highly respect what you're saying. I know it's difficult to communicate what you are saying without illustrations and the math (though the math is probably well beyond most shooters here). But I feel your effort is probably appreciated by some others than just myself; at least, I hope so. I've saved a copy of this for rereading to try to fully assimilate all that you've pointed out.
 
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Seating the bullet farther away from the lands decreases the time the bullet spends traveling down the tube. That's gotta help in keeping everything happening in that first mode.
 
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Seating the bullet farther away from the lands decreases the time the bullet spends traveling down the tube. That's gotta help in keeping everything happening in that first mode.

I am not agreeing or disagreeing - why do you think that increasing jump reduces barrel time?

Are you thinking that if the bullet starts further back, it gets a running start and enters the rifled part of the barrel with some initial velocity? How does it work?
 
Two examples with made up numbers.
1) Bullet on the lands. The bullet goes from 0 to 2000 fps in 2 feet. Its average velocity in the tube is (0+2000)/2. Average velocity in the tube is 1000 fps. It is in the barrel for 0.002 seconds
2) Bullet seated deeper. Jump. The running start. It hits the rifling at 1000 fps but exits at the same 2000 fps. Its average velocity in the tube is (1000+2000)/2. Average velocity in the tube is 1500 fps. It is in the barrel for 0.001333333333333 seconds.

EDIT: I can conceive that if there were a very long freebore, enough that the bullet has completely exited the case and becomes unsupported before contacting the lands, the bullet could enter the rifling with a yaw and create other problems.
 
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Not sure where to begin...but jump testing is something we all see a lot, and it typically the discussion goes back to bullet timing/barrel vibrations/etc...along those lines.

one thing that has always hung in my mind is "forgiving" bullets...how are bullets "forgiving" if the whole idea of load dev, is based upon timing up when the bullet exits the muzzle with some movement/vibration pattern...do forgiving bullets not make the barrel move/vibrate? (obviously not the case) do forgiving bullets not deflect the same when exiting a moving barrel? self correct? i dont have an answer, just questions...

is it possible that instead of barrel timing/muzzle exit, all seating depth needs to be is "good enough to hand off the bullet straight"? ie; Bullet A, gets a clean/straight hand off due to bullet shape and chamber spec relationship from .005" to .150" off...basically it gets a good hand off anywhere having the bearing surface out of the neck because that "forgiving" design allows it so...while maybe Bullet B, has less agreeable relationship with chamber design, and it doesnt get a clean/straight hand off from .005" to .020" off...gets straight from .025 to .050"...then its iffy again past that...again, just illustrative examples

all of this also assumes the reloading practices and chambers have the correct relationships...bad reloading practices or bad chamber specs/chamber jobs is another thing...basically sound reloading practices (consistent doesnt always equal sound...if youre making burs in brass, deforming bullets in seating, etc...even if you do it every time...its not good for results)

im sure anyone who has done much reloading/shooting knows certain bullets just shoot in a lot of barrels...its almost hard to get them not to...SMKs, Berger BT Target (better than hybrids in my experience)...Hybrids and Hornady ELDs are way better than most also

For actual shooting scenarios where this has shown up for me...i had a 223 barrel a while back...shooting 80 VLDs. I did powder tests, i did the Berger jump tests (the off the lands version i think from .020 to .120 or whatever it was)...and everything was basically 3/4" @ 100...some groups around 1/2", some closer to 1", but everything in a decent sample size...3/4", which is not a pretty looking group shooting small .224 bullets

Buddy of mine said "try them .005" off"...so i did, groups instantly cut in approx. 1/2...went to shooting a lot of ragged holes groups on the regular in the 1/4-5/8" range...and it did this for a pretty long time, until the lands wore away...it basically became a 3/4moa system again over time

at the same time...ive had A LOT of barrels over the years...easily north of 20 since 2015...where i shot forgiving bullets...SMKs, Berger BT target/juggernauts/hybrids...and in those barrels, i cant find a REAL seating depth difference...i can seat a Berger BT Target (what i use most the last few years), anywhere from .020 to .100 off, and every barrel will shoot them basically the same for relevant sample sizes...i cant shoot the difference day to day, but i know everyone on the internet shoots better so maybe they can lol

so my question goes back to...if jump (and powder charge) have to do with barrel timing/vibrations/muzzle movement...why do certain bullet designs seem to ignore this? barrel has still got to be moving

is it possible that the amount of jump is just a pass/fail scenario? in that it gets handed off clean/straight or it doesnt in relation to how the case/bullet reacts with the chamber geometry

of course, it could also be possible that shooting from a bipod/rear bag...the barrel vibration/movement variation is so small, i cant reliably/consistently see it in the results...and bullets being handed off crooked is all i can shoot accurately enough to see on target...maybe?

and yes...ive seen peoples small sample testing that show "clear winners"...i have stacks of my own also. I also have stacks of load dev where i shot "forgiving" bullets across 2-5 grain charge windows and .100" jump windows that didnt move significantly at all...possible can barrels be oscillating within a similar window at all times, and the variation seen on target is just how straight the bullet was handed off and leaves the bore?

ive also seen the barrel time info/papers/data....but i still hang up when that doesnt apply or show up on target...

curious to hear some other thoughts and experiences...lots of nuance and noise to weed out in these topics so hopefully i got my thoughts across clearly

id also like skip over any "i shot 5 rounds of A and 5 rounds of B, and A was .1" smaller so A will always be better..." ...we all know how that would hold up lol

Side Note: outside of the "unforgiving" VLD design in that 223 that showed a clear difference...i dont see the large shifts/changes a lot of people seem to see from small jump variations, or powder for that matter...its not like .020 is 1/3moa for 10 shots, and .030 is 1.25moa for 10 shots...if i do see large variations, when retested, they dont hold up, typically attributed to an outlier or i shanked one...my variations are always very small, with lots of overlap, and larger samples blend the differences together...simply put, my barrels/rifles dont consistently (aka the vast majority of the time) shoot better than 1/2" or worse than 3/4" for large sample sizes...no matter what i change in my loads with the "forgiving" bullets (within reason, using good components and known reliable combos)...yes yes, ive shot a ton of ragged hole groups like everyone else, but theyre small outliers, not the overall average everyday
Very good series on seating depth on Precision Rifle Blog. I’ve attached a link to the summary, but the whole series is a very good read.


I have read over the years that kissing the lands or a light jam provides the best precision, but I cannot recall one single time where that has been true for my rifles. I seem to have the most consistent results at .050” off, or even more.

John
 
Except that he begins his article with a nonsense premise that lands erode at a rate of .004” to .007” per 100 rounds.
I’ve never measured erosion rate so took him at his word. What is the erosion rate for say, a 6.5 Creedmoor?

John
 
I’ve saw 003-.005 over a 200ish round match typically back when I measured…that’s 200 rounds fired quickly in a 90 sec par time setting and usually in matches were temps were 80-100°
 
Since my 6.5 PRC barrel was new to now with 957 rounds fired, the throat has eroded .039", which is just a hair over .004" per 100 and the loads were all in the upper end of stout loads . . . like ~58,000+ psi. For my .308 Krieger barrel, after 1402 rounds fired, the throat has eroded .020", which is and average of .0014" of throat erosion per 100.

The more powder you burn and at higher pressures and higher barrel temperature, the faster the throat will erode.
 
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There is a strong relationship between cyclic rate - temperature, and the erosion rate of barrels.

All other things being equal, slowing cyclic rate or lowering material temperatures will increase barrel life.

If we want to see a dramatic example, take a look at the life of an AR15 match barrel used only for Mid Range matches, versus the identical design being used in XTC matches where their life is much shorter due to the tempo of the rapid strings.

The Mid Range Match runs 15 to 20 shots plus sighters at a tempo where they give roughly 1 1/2 mins per record shot. Most experienced shooters will shoot fast if they catch a condition and they are worried that it won't last, but on average it doesn't take them 20 minutes to fire a 20 shot string, and yet their barrels are not too hot to touch when they are done. These are all single fed shots but not near as fast as the rapids.

The XTC rapid fire strings are where the majority of the damage occurs because those are fired at a rate of 10 record shots in 60 seconds sitting and 70 seconds prone, with a short break between two strings where the barrel is usually still pretty hot for that whole second string in places where electronic targets mean the pits don't need to clean up the targets.

The significance of erosion or wear for barrels can mean different things to different folks. Checking the distance for a given bullet to the lands, versus where the rig won't hold a group can be an interesting debate. One would assume that an XTC rifle will be worn by showing a poor 600 yard line group first, but in reality it is usually going to show in the second rapid fire string at 300 or 200 before it shows up at 600, due to the heat.

Just watching those guns in terms of seating depth and wear, and versus 600 yard performance will make one really think about the significance of seating depth and if it really matters. YMMV
 
Two examples with made up numbers.
1) Bullet on the lands. The bullet goes from 0 to 2000 fps in 2 feet. Its average velocity in the tube is (0+2000)/2. Average velocity in the tube is 1000 fps. It is in the barrel for 0.002 seconds
2) Bullet seated deeper. Jump. The running start. It hits the rifling at 1000 fps but exits at the same 2000 fps. Its average velocity in the tube is (1000+2000)/2. Average velocity in the tube is 1500 fps. It is in the barrel for 0.001333333333333 seconds.

EDIT: I can conceive that if there were a very long freebore, enough that the bullet has completely exited the case and becomes unsupported before contacting the lands, the bullet could enter the rifling with a yaw and create other problems.
According the Gordon's, 308 with 41.52 grains of IMR 4064, Sierra 175 SMK from COL 2.700 to 2.850 in steps of 0.025 in.

1674507957071.png


The purple line is velocity - note, velocity is not linear over time so using average velocity to model barrel time doesn't work.

cartridge length / barrel time (ms) / max pressure (psi) / bullet lead time (ms)

2.700 / 1.3063 / 58696 / 1.2484
2.750 / 1.3270 / 56117 / 1.2701
2.800 / 1.3471 / 53762 / 1.2920
2.825 / 1.3570 / 52659 / 1.3030
2.850 / 1.3668 / 51602 / 1.3142

As the cartridge gets longer (that is, there is more space in the case and the bullet gets closer to the lands) the max pressure drops by 13% and barrel time increases by about 60.5 microseconds or about 4%. Time spent in the lead increases by 65.8 microseconds. It appears that the barrel time is inversely proportional to peak chamber pressure. FWIW, Gordon's muzzle velocity estimate is about 100 FPS low. Gordon's is not the only internal ballistics modeler but it isn't terrible.

You assume that with a long jump the bullet hits the lands at some high FPS. I do not think that is how it works. If you have measurements for internal ballistic velocities, please post them. If you are relying on some authority, please post a link.
 
cartridge length / barrel time (ms)

2.700 / 1.3063
2.750 / 1.3270
2.800 / 1.3471
2.825 / 1.3570
2.850 / 1.3668
Look at that. Long cartridge equals more time in the barrel. What you have posted directly supports my statement. Thank you.
 
I’ve saw 003-.005 over a 200ish round match typically back when I measured…that’s 200 rounds fired quickly in a 90 sec par time setting and usually in matches were temps were 80-100°
Proves the know it all wrong already . He just came up with a random number of .001" per 100 to sparkle us with bullshit . Thank you for giving real world facts .
 
Look at that. Long cartridge equals more time in the barrel. What you have posted directly supports my statement. Thank you.
Long cartridge = more barrel time cause it’s slower. In his example he held the same charge weight for all load lengths. 13% less pressure for the long one = xxx? Slower MV = longer barrel time.
 
Not to go off on a tangent here...

But as an aside, part of the problem with all of these reloading "theories" is that guys are basing their conclusions only off of what they can produce, or are capable of, and then putting that out into the ether like it's fact (when it's not). And, the list of debunked "legacy reloading theories" only seems to grow once guys become more critical of them it seems.

IDK about anyone else, but I haven't rolled up to any ranges with full-on mechanical rests that can completely lock one's gun in place to do any real empirical testing of anything? Because if there's a human involved, then shit is getting fucked up...

Like earlier when @morganlamprecht mentioned F-Class guys are shooting off of uber-solid joystick front rests and specialized burly rear bags... which is having a huge effect on what they see, versus a more PRS-ish style shooter using a bipod like a Harris/Atlas and a Gamechanger or something for a rear bag...

My hunch is: most times, the joystick front rest and Dima bag probably have more to do with the size of the groups than the variation in seating depth... So it becomes hard to decide if any valuable information in so far as it pertains to one seating depth being better than another can be gleaned.

So if an F-Class guy, even if he's shooting the same bullets as me, tells me what seating depth yielded the best groups for him... to me, it means almost nothing, I still have to shoot like I shoot, off my shit, to know anything.
 
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Don't take this next statement as a condemnation of GRT or QL because my opinion is that they are still very useful models for playing with wildcats and estimating effects of changes. However...

Here is the thing about using models like GRT or QL to try and get details like pressure/velocity versus seating depth and thus making conclusions about OBT theory.

If you actually test the effect of fine seating depth in a sweep while holding the charge constant, the velocity drops with deeper seating depth, it does not climb. It goes counter to the model and counter to many opinions on the internet.

Here is an example of a Dasher with a wide seating depth sweep that goes from touching the lands to 0.080 deeper. Notice the velocity is not increasing and is in fact dropping from 2980 down to 2950. Try it yourself. YMMV

1674583421210.png
 
Increasing jump beyond ~.010 has very little reduction in pressure the further away you get from the lands. If one is within that zone and as one increases jump there is a significant reduction in pressure. The statement that "increasing jump tends to reduce pressure is true, but it's not as though it's some kind of linier reduction; it just depends on where you're reducing the jump from.

Am still leaning and found Jeff Siewert's book Ammunition Demystified very helpful.
 

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Don't take this next statement as a condemnation of GRT or QL because my opinion is that they are still very useful models for playing with wildcats and estimating effects of changes. However...

Here is the thing about using models like GRT or QL to try and get details like pressure/velocity versus seating depth and thus making conclusions about OBT theory.

If you actually test the effect of fine seating depth in a sweep while holding the charge constant, the velocity drops with deeper seating depth, it does not climb. It goes counter to the model and counter to many opinions on the internet.

Here is an example of a Dasher with a wide seating depth sweep that goes from touching the lands to 0.080 deeper. Notice the velocity is not increasing and is in fact dropping from 2980 down to 2950. Try it yourself. YMMV

View attachment 8056431
ive found the same...holding everything the same except seating depth, the deeper a bullet is seated (more jump), the more velocity bleeds in everything ive tested it
 
Don't take this next statement as a condemnation of GRT or QL because my opinion is that they are still very useful models for playing with wildcats and estimating effects of changes. However...

Here is the thing about using models like GRT or QL to try and get details like pressure/velocity versus seating depth and thus making conclusions about OBT theory.

If you actually test the effect of fine seating depth in a sweep while holding the charge constant, the velocity drops with deeper seating depth, it does not climb. It goes counter to the model and counter to many opinions on the internet.

Here is an example of a Dasher with a wide seating depth sweep that goes from touching the lands to 0.080 deeper. Notice the velocity is not increasing and is in fact dropping from 2980 down to 2950. Try it yourself. YMMV

View attachment 8056431
ive found the same...holding everything the same except seating depth, the deeper a bullet is seated (more jump), the more velocity bleeds in everything ive tested it
Now....why does velocity decrease?
I suppose it could be that a portion of the explosion is now able to slip by the projectile before a good seal is created against the bore.
 
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Not to go off on a tangent here...

But as an aside, part of the problem with all of these reloading "theories" is that guys are basing their conclusions only off of what they can produce, or are capable of, and then putting that out into the ether like it's fact (when it's not). And, the list of debunked "legacy reloading theories" only seems to grow once guys become more critical of them it seems.

IDK about anyone else, but I haven't rolled up to any ranges with full-on mechanical rests that can completely lock one's gun in place to do any real empirical testing of anything? Because if there's a human involved, then shit is getting fucked up...

Like earlier when @morganlamprecht mentioned F-Class guys are shooting off of uber-solid joystick front rests and specialized burly rear bags... which is having a huge effect on what they see, versus a more PRS-ish style shooter using a bipod like a Harris/Atlas and a Gamechanger or something for a rear bag...

My hunch is: most times, the joystick front rest and Dima bag probably have more to do with the size of the groups than the variation in seating depth... So it becomes hard to decide if any valuable information in so far as it pertains to one seating depth being better than another can be gleaned.

So if an F-Class guy, even if he's shooting the same bullets as me, tells me what seating depth yielded the best groups for him... to me, it means almost nothing, I still have to shoot like I shoot, off my shit, to know anything.

Soooo what you’re saying is that all these statistics being used to “debunk” reloading theories are just a justification for incompetence.
 
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Now....why does velocity decrease?
I suppose it could be that a portion of the explosion is now able to slip by the projectile before a good seal is created against the bore.
I cannot tell you one way or the other.

There are rough (empirical) values for the beginning of the combustion process in both models, and the main modifier would be for jam versus jump.

Everyone, including myself, rests easier when we can explain the physics of a model to the point where we can make predictions and extrapolations.

But then there is the reality of "good enough for government work".... it isn't just a joke for when work is sloppy or incomplete, there is some truth to it....

Mother Nature is cruel and demands a much bigger budget when it comes to chaotic topics like friction, combustion, and ballistics. When the budget covers the basics of the contracts, there is usually not enough left over to answer the basic nature questions. As long as the models are useful for getting the job done, we marched forward using fudge factors.

That leaves behind unanswered questions for the next generation of investigators and PhD candidates to answer.... YMMV
 
Look at that. Long cartridge equals more time in the barrel. What you have posted directly supports my statement. Thank you.
I plan to do a test. I have a bunch of 308 already loaded to 2.820 all with 41.52 +- .02 gr IMR 4064 powder charges. I am going to take 10 rounds and reduce the COL to 2.810, another 10 to 2.800, and another 10 and to 2.790. I will hang paper at 500, shoot groups, and see what happens. 2.820, 2.810, 2.800, 2.790. Since I am an average shot, I don't expect much but - we will see.

BTW, I can't see 600 from my favorite bench and 700 is a bit long for groups. If the board at 500 is too messed up, I will hang paper at 400.
 
there will be a winner

The real question is does the winner still win consistently/majority of the time when repeated

What bullets are you shooting?

If not doing round robin where u shoot 1 of group A, 1 of group B, 1 of group C, etc…pay extra attention to barrel heat as you work through the groups
 
If you can get a chrono on those tests at the same time as shooting the groups, it would also add to the story. It would have to be something other than a MS so it cannot affect your tune. Good Luck.
 
Purchasing LabRadar years ago was a huge time saver for me

Being able to shoot for actual POI and get velocity on everything was really convenient
 
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I plan to do a test. I have a bunch of 308 already loaded to 2.820 all with 41.52 +- .02 gr IMR 4064 powder charges. I am going to take 10 rounds and reduce the COL to 2.810, another 10 to 2.800, and another 10 and to 2.790. I will hang paper at 500, shoot groups, and see what happens. 2.820, 2.810, 2.800, 2.790. Since I am an average shot, I don't expect much but - we will see.

BTW, I can't see 600 from my favorite bench and 700 is a bit long for groups. If the board at 500 is too messed up, I will hang paper at 400.
To really do this in what I would consider to be the most effective way would be to start with the bullets very near the lands. I find that by using a dry erase marker and draw all over the projectile on a dummy round. Then I chamber that round. Observe that the marker has been gouged, wipe the round clean, seat that bullet a small amount, draw on it, chamber, repeat until I can no longer see any evidence of the dry erase marker being disturbed, but just barely. At that point, I know that the bullet is maybe 0.001" from the lands. Note that when I first get started, sometimes that bullet sticks in the lands and is pulled out of the casing. I do remove the ejector before starting and try to load the bullet as gently as possible by hand rather than stripping from the magazine and gently remove it from the chamber.

Then I back away from the lands 0.002" at a time and shoot 10 shots at each new depth. I'd clean the rifle and let it cool in between each set of 10. Or better yet, do a round robin style of shooting

Now, it might be that you are already at the optimum seating depth for that particular arrangement of components.

As stated previously, I seek for max safe velocity (pressure signs are not there but just barely not there anymore) and then find the seating depth.

EDIT for spelling.
 
Now....why does velocity decrease?
I suppose it could be that a portion of the explosion is now able to slip by the projectile before a good seal is created against the bore.

Blow by as well as time.

You have more time from the moment bullet starts moving before it’s engraved.

That means more time for everything. Blow by, energy transfer in the chamber and barrel, etc etc.
 
Without any sort of explanation of what and how those pressures are being measured, the pics are fairly meaningless.
For example, is this the pressure as measured from the time ignition begins?
Is this pressure as measured with the bullet STARTING at some distance off or not?
ETC
 
Don't take this next statement as a condemnation of GRT or QL because my opinion is that they are still very useful models for playing with wildcats and estimating effects of changes. However...

Here is the thing about using models like GRT or QL to try and get details like pressure/velocity versus seating depth and thus making conclusions about OBT theory.

If you actually test the effect of fine seating depth in a sweep while holding the charge constant, the velocity drops with deeper seating depth, it does not climb. It goes counter to the model and counter to many opinions on the internet.

Here is an example of a Dasher with a wide seating depth sweep that goes from touching the lands to 0.080 deeper. Notice the velocity is not increasing and is in fact dropping from 2980 down to 2950. Try it yourself. YMMV

View attachment 8056431
I finally found what I've been looking for for a long time in that 1965 Lloyd E Brownel's report that MarkyMark07 posted a link to. It better helped me understand why many competitive shooters like to seat their bullets into the lands as well as what's going on with maximum chamber pressures changing (as you've described) with seating depths.

I've always wondered if there was enough blowby along with increase in chamber volume to override Boyle's Law effect for increased seating depth lowing case volume for more pressure . . . as QuickLoad and GRT illustrates. It's very apparent to me now that it does, but only within range of possible seating depths; though I'm sure the design of a bullet and the chamber have a big influence on what the range looks like.

Here's a graph and the part of that report that pretty much says almost all I need to know about it as a precision reloader:

Seating Depth vs Pressure graphic.jpg


Seating Depth vs Pressure.jpg
 
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I finally found what I've been looking for for a long time in that 1965 Lloyd E Brownel's report that MarkyMark07 posted a link to. It better helped me understand why many competitive shooters like to seat their bullets into the lands as well as what's going on with maximum chamber pressures changing (as you've described) with seating depths.

I've always wondered if there was enough blowby along with increase in chamber volume to override Boyle's Law effect for increased seating depth lowing case volume for more pressure . . . as QuickLoad and GRT illustrates. It's very apparent to me now that it does, but only within range of possible seating depths; though I'm sure the design of a bullet and the chamber have a big influence on what the range looks like.

Here's a graph and the part of that report that pretty much says almost all I need to know about it as a precision reloader:

View attachment 8058647

View attachment 8058648
Now recognize just how far off the lands you have to be for pressure to increase.
It flattens at only half an inch off the lands in that super graph. Good luck doing that in most rifles. Most rifles even compressing powder can’t.

Pistol the seating depth has a large effect on chamber volume behind the bullet and has much larger swings. Rifles with their small bullets and big cases have a much larger ratio of volume to overcome.
 
Oh yes, @spife7980 raised an important safety issue. Important to note for students and rookies.

Be very careful with pistol cartridges with respect to seating depth.
 
This is like some existential philosophical discussion late at night in the college dorm back in the early 70’s we used to have.
Somewhat mind altering yet no solid conclusions that are verifiable.
I love it.