Trying to understand seating depth and accuracy

[Flakbait]

Happy New Year!

I am trying to wrap my finger around the phenomenon of bullet seating depth changes and accuracy. I understand that changing the seating depth of say a Berger VLD bullet (secant ogive) can dramatically affect the accuracy of groups assuming no other changes in load are made.

Fortunately for owners of Remington factory rifles with extremely long barrel throats, other bullets with a different ogive profile (tangent) like the SMK are less sensitve to bullet seating depth changes allowing decent accuracy loading from the magazine. I think my Rem 700 bullets travel well over .100 of an inch to hit the rifling of the barrel.

Does altering the bullet seating depth of a particular load by say .010 change the pressure curve of the burning powder (assume no compressed loads) and thus affect the timing of bullet exit from the barrel?

Will changing bullet seating depths change the muzzzle velocity (assuming no compressed loads)?

I assume compressing loads can dramatically raise peak pressures leading to unsafe conditions.

Another explanation is that .010 difference in the length the bullet travels will affect the bullet exit timing.

I am searching for a reasonable explanation...perhaps I should dive into Bryan Litz's Applied Ballistics book sitting on my bookshelf.

 

[Tripwire]

Re: Trying to understand seating depth and accuracy

Here's where I stand with seating depth, and it's function with helping, or hurting "accuracy".

First of all I subscribe to the OCW load principles and the effects barrel harmonics play. We need not go into depth on that beyond there being a "tuning fork" type shock wave that travels the length of the barrel, back and forth several times during the bullet's travel, before the bullet exits at the muzzle.

When the shockwave is at the chamber end of the barrel the muzzle is at its calmest state. When the shockwave is at the muzzle then the muzzle is occilating. The only thing barrel profile controls is the <span style="font-style: italic">amount</span> of occilation present. Heavy barrels move less than skinny barrels.

During load development you look for "nodes", both accuracy nodes, and scatter nodes. That's all self explainatory per the above. You reach these nodes, in the rough, with incremental changes in powder charge, which are also incremental changes in pressure level, which are incremental changes in bullet velocity, and thus incremental changes in the speed at which the bullet travels down the barrel. Evaluating the test results puts you into a zone of pressure and velocity that's acceptable...which is also putting you where the shockwave is not AT the muzzle.

To refine the load you adjust the seating depth, which in the increments I adjust, is NOT a factor of adjusting the pressure to an appreciable degree. It is however, a factor of adjusting the length of time the bullet is in the barrel.

Since the shockwave is slinging back and forth faster than the bullet is traveling there is a very short and narrow window we know as an accuracy node (when the bullet exits with the shockwave at the chamber end of the barrel). Adjusting a few thousandths one way or the other either lengthens, or shortens, the bullet's barrel time just enough to put the timing of the exit of the bullet from the muzzle when the shock wave is the absolute furthest from it.

I see this easily with an OCW workup when my OCW is found and the groups are just a little bigger than I think they should be. By shortening the barrel time of the bullet by seating longer, or by lengthening the barrel time by seating shorter, I can adjust the timing of the exit of the bullet to put it when the muzzle is at its calmest state, and the result is smaller groups with that load.

 

[MALLARD]

Re: Trying to understand seating depth and accuracy

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Flakbait</div><div class="ubbcode-body">

I am trying to wrap my finger around the phenomenon of bullet seating depth changes and accuracy.

Does altering the bullet seating depth of a particular load by say .010 change the pressure curve of the burning powder (assume no compressed loads) and thus affect the timing of bullet exit from the barrel?

Will changing bullet seating depths change the muzzzle velocity (assuming no compressed loads)?

I assume compressing loads can dramatically raise peak pressures leading to unsafe conditions.

Another explanation is that .010 difference in the length the bullet travels will affect the bullet exit timing.

I am searching for a reasonable explanation...perhaps I should dive into Bryan Litz's Applied Ballistics book sitting on my bookshelf. </div></div>

this is a pretty good thread already , hope i dont ruin that

as far as i understand adjusting bullet seating "only" effects pressure , it doesnt really have a big effect on exit times.

a .010 difference like tripwire said , does nothing to effect the safety of your loads , unless your loads were hot, like the edge of overpressure , that then with tempreture changes could lead to overpressured ammo. But , its an unlikely senario i think because you wouldnt be playing with a load that hot , i would hope.

most people agree you can reduce your vertical by adjusting seating depths , but thats not to say your actually making the group smaller , you might have some horizontal you can't squeeze out. So weather you've really shifted your exit times , is unlikely. To go one step further , i never noticed that on my pressure trace machine but i must admit i never tested that indepth , but what i have seen was minimal

 

[Tripwire]

Re: Trying to understand seating depth and accuracy

Okay, let me take another stab at this, speaking only from the OCW angle, as that's the leg I currently stand upon.....and likely will until a better load development process is invented.

Per a bullet's barrel time, the shockwave, and the rudimentary physics of all that:

During a load work up with OCW, I sorta ballpark a starting point without getting too anal with figuring out the percentages like my buddy Dan Newberry does. This is contrived via running my components and a WAG on bullet seating through QL. I'm interested only in the fastest accuracy node that is still within a safe limit of pressure, so I'll typically back down enough to find the second fastest accuracy node, work up from there to the fastest accuracy node, and see the pressure above that that sets the limit.

So what I end up with as cold hard on paper fact; is a scatter node just prior to the second fastest accuracy node, a scatter node above that, the fastest accuracy node, the scatter node above that, and the pressure limit sign telling me to stop there.

This translates into; first) the shockwave being at the muzzle or near the muzzle to start. Either at it, just approaching it, or just moving away from it. Second) is the shockwave at a point away from the muzzle which give me the lower accuracy node. Third) is the shockwave approaching, at, or moving away, from the muzzle producing the scatter node just prior to the fastest accuracy node. Fourth) is the shockwave again away from the muzzle at some point in its travel producing the highest velocity accuracy node. Fifth) is the shockwave again approaching the muzzle, at it, or moving away from it in conjunction with a too high pressure situation...usually for me a flat primer or a sticky bolt.

What all that shows us, on paper, is how the shockwave is affecting the muzzle by what the POI of the bullet is over the spectrum of the test.

Now understand this, in a medium sized case capacity, which I consider a 30-06 to be, the accuracy node is likely going to cover a whole grain's worth of powder...IOW, .5 grains above and .5 grains below the center of the node. The analysis of the test via bullet impact shows that the POI is the same for the whole window of one grain of powder. Adjusting bullet seating depth by a mere .005" or .010" total will in no way produce a pressure change that can over ride the tolerance to one whole grain's worth of pressure difference found in the accuracy node.

But, that change in seating depth, in the context of the speed by which the shockwave is moving back and forth, WILL have an effect on the point in time at which the bullet exits the muzzle, as verified countless times by my own load development since I switched to OCW nearly 10 years ago.

We find a pressure zone that works and that is safe by changing the level of powder charge. And via the OCW doctrine, we can refine the real accuracy produced by making very small changes in the time at which the bullet exits the muzzle, by addressing where the shockwave is in relation to the muzzle, and by putting as much distance as possible between the shockwave and the exit time of the bullet. Think of it as something similar to bracket racing with a bullet. Too slow and it won't work, too fast and it won't work, just right and you nail it down for the prize.

I discussed this same subject with Dan Newberry over lunch one day and he completely agrees with me how bullet seating affects the OCW load work up. This is why I've no fear at all to work a load up in .1 or .2 grain increments because it gives me a VERY clear picture of the barrel harmonics, and why I typically never have to adjust a seating depth for group shrinkage more than 8 or 10 thousands....and that would be a worst case scenario.

It's basic simple physics, but you have to slow everything down in your mind to see how it works........

 

[kineticmedic]

Re: Trying to understand seating depth and accuracy

Thanks for the explanation. I need to read more on OCW.

 

[Trapshooter12]

Re: Trying to understand seating depth and accuracy

Trip, that is a great explination.

 

[Tripwire]

Re: Trying to understand seating depth and accuracy

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ubergeek</div><div class="ubbcode-body">Thanks for the explanation. I need to read more on OCW. </div></div>

Just talk to Dan himself......link

 

[X-fan]

Re: Trying to understand seating depth and accuracy

Great post Tripwire!

I would like to add some observations from pressure tested data (Oehler 43 and RSI PTrace).

Moving the bullet from 10 thou out of the rifling to touching the rifling raises pressure 4-8Kpsi.
Also of interest is the change to the beginning and area under the pressure curve. Pushing further into the lands ramps initial pressure quicker, but does not significantly add to overall peak pressure.

Up until today I (perhaps wrongly) assumed VLDs like the bigger kick in the pants.

 

[Tripwire]

Re: Trying to understand seating depth and accuracy

I'm sure there's an exception to the rule, if you can even call it that...and my conclusions are losely based on what I've seen for myself. Going OCW has worked very well for me, but like Dan too, I'm in no way shape or form a benchrest shooter.

It bugged the hell out of me that a seating adjustment of something like only .005" could dramatically improve a load's group, as that is an almost negligable increase or decrease in total case capacity and the resulting pressure increase/decrease would be almost certainly not a factor. With the help of my Dad, who spent thirty five years teaching chemistry and physics, we concluded it had to be a function of barrel time and timing the exit of the bullet in relation to the position of the shock wave. Newberry concures.

Per VLD's, I don't work with them, though they've tweaked my curiosity here lately. Vmax, TSX, and SMK's are the bulk of what we shoot here, and mag box constraints tend to limit touching lands in most of our sticks. I keep recalling a factory 700 BDL in .270 winchester that I had a couple of decades ago. One of the loads that rifle would REALLY shoot was obviously just a high odds stroke of pure luck. Can't recall much of the pariculars per powder, brass, ect, but the bullet was the 90 grain Sierra HP, short fat little bastard that when seated with a whole bullet diameter's length in the neck it had a jump to the lands of .250", a whole quarter of a damn inch. Touching the lands isn't all it's cracked up to be.

 

[Mike Casselton]

Re: Trying to understand seating depth and accuracy

To add to Tripwires 270 oddity. 16 years ago I bought a Remington Varmint Special in .243. The gun looked brand new, but the barrel had evidence of it being fired.

Got it home, cleaned up the barrel, adjusted the trigger and removed the cosmoline looking stuff in and on the action. Mounted a trusty scope on it and made up some test loads to take to the range. Over three or four sessions, I tried all kinds of things to get it to shoot. It just didn't show any promise. I couldn't even reach the lands with a 100 gr bullet.

Next step was to do a bedding job so out came the dremel to make room for the bedding compound. The bedding came out perfect so I reassembled it and tried all kinds of loads again. Nothing. Same as before 1.5 to 2.5 inch groups. Since I couldn't reach the rifling I got pissed off and loaded some rounds with Varget and a 55gr Nosler BT (same as before) and then screwed the seater stem down until the base of the bullet was even with the bottom of the neck. It looked goofy with that tiny bit of bullet sticking out of the case, but I had nothing to lose. In fact, I was beginning to understand why a nearly new rifle was in the used rack.

Well guess what. The first five shot group was right at 3/10 of an inch. So was the second, third and fourth groups. I used the other five to smack some jackrabbits.

This gun went against everything I had been taught about loading for accuracy as far as seating depth was concerned. Every load I ever shot out of that gun after that day was seated without moving the seater die. 55 and 70 gr Nosler BT's, 58 and 65gr V-Max's, 70 gr TNT's and some other Hornady 70 gr SX's. Over the time I owned that gun I refused to move the seater stem and that gun kept shooing under 1/2". A buddy of mine ended up with it just before I moved to FL. I wish I still had it...

 

[X-fan]

Re: Trying to understand seating depth and accuracy

Trip,

Pretty much the only setups that I run are into the lands are VLD rigs.

I did have a factory 300Win Sendero that refused to shoot better than MOA until I jammed the bullets and redialed the load. Then it shot under 3/4 inch at 200 yards. I suspect this had to do with the crappy factory chamber (6 thou runout built in!), which mattered not as I proceeded to shoot that barrel into oblivion. The new one shoots perfectly 50 thou off the rifling.
God I love the 300Win!

I run all of my match rigs (hunting too) 50 thou off the rifling...I find that either they shoot like that or something is wrong with the rifle. Just dial the load and shoot.

Rifles like my 6mbr shoot tighter, but the tuneup is much more critical in the quest to maintain peak (hair splitting)accuracy. As the throat moves forward accuracy will suddenly vaporize where as the 0.050 setup could care less what the throat did for hundreds of more rounds.

In terms of forgiveness and repeatability I feel most of us shooting a 308 (or other high volume cartridge) are better off staying well off the rifling, but that is just me.

Peace, and Happy New Year!

 

[Tripwire]

Re: Trying to understand seating depth and accuracy

All interesting stuff...

Don't however let this thread erode too much into one about seating in the lands or not being right or wrong, there's plenty of that already, around here.

I'm in no way an expert on bullet seating, or reloading for that matter...just elaborating on my own experiences, and the stuff that's proved out for me over a broad enough spectrum to call it GTG. The success of my shooting, now, is defined by a clean kill from a single hit on something I want to eat, and hitting other things, during practice, when and where I want to. The only paper I shoot is during load development. My judgement on accuracy for my own purposes uses 1 MOA as the keep or kick mark, I don't want to fool with one that produces worse than MOA. I usually build a rifle around a specific bullet, and that usually generates something WAY better than MOA.

I've also found though, that using OCD-OCW (obsessive compulsive disorder optimal charge weight) will usually bring a plain old POS factory barrel right in there with the rest of them, case in point is my son's 243 BDL wood stocked 700, 80's era. That rifle is bedded and floated, and has good glass on it....but nothing else special about it. It shoots 87 Vmax's and 85 TSX's stupid good. OCD-OCW is: using .1 grain increments of charge increase in small cap cases, using .2 grain charge increases in larger cap cases, using seating depth changes as small as .002", and actually seeing the nodes form and go away on paper.

Would enjoy hearing about other's experiences that contradict mine. That's how we all learn something. Hell, that's how I got into OCW 10 years ago when I read something this dude Dan Newberry wrote about a load development practice that worked very well in just about any rifle. I took the bait and ran with it. Figured out later Dan lived within a half hour of me and that's just been icing on the cake having him as a friend.

 

[X-fan]

Re: Trying to understand seating depth and accuracy

Unfortunately my findings are about the same Tripp.

I have a bud that shoots the 6mmbr like a demon possessed and I know for a fact he sees things different. He talks in terms and goals of 0.10 groups at 100 and sub 3 inch groups at 600 so this is a bit of a different level and has nothing to do with hunting or tactical shooting for that matter.

He works up a node with known powders and charge ranges and if he doesn't like what he sees he will rework the node 5 thou deeper and so on. He maintains smaller nodes mean more consistent shooting. I have seen him shoot said 0.1 groups more than once and ladders that measured about 5/8 inch.

I have tried this with my rifle (identical) and to be honest even with my 12-42 BR NF I just don't have the patience to group well enough. If I see sub 1/4" I just put the crosshair on the dot and pull the trigger, usually get caught by a bit of weather and get a 0.3 or whatever. It gets a bit tedious and I suppose when you get right down to it I guess I don't really much care.

The other thing about a setup like this is you are constantly chasing it ever 200 rounds or so..Chase the rifling then bring the powder charge back up to speed then retest.

For me, I just want to shoot.
Sub 1/4 moa doesn't mean crap when I get beat by guys that have 1 MOA rifles that can read the wind better than I do.

Peace

 

[Leaddog]

Re: Trying to understand seating depth and accuracy

Tripwire- Excellent post up! Thanks.

I use Quickload and OCW for working up loads. I have two custom 10 twist .308's I use for F Class at 600 and 1000 yards. I shoot Berger 185 LRBT's with good success. I can load for a slower node at 600, but I cannot reach the next higher node for 1000 yards without seeing pressure signs. I have an in-between nodes load that is tweaked by ES/SD using the best seating depth. It shoots well, but in cold weather it is slower than I like at 1000 yards. I use Varget.

In my case, do you suggest going with another powder like N550 in the colder weather to get the velocity up closer to that next higher node without the pressures that Varget generates?

I appreciate your feedback. Thanks.