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Seating depth, what’s acceptable variation?

NJRaised

Sergeant of the Hide
Full Member
Minuteman
Feb 7, 2021
252
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Port Murray NJ
Running a rcbs matchmaster micrometer seating die (223). My load is right on the verge of being compressed. You can hear a kernel or two of powder when seating.

Out of my last batch of 100 rounds , about 70 rounds are right at my desired seating depth. Another 20 rounds or so, at within .001”, and then I got a handful that are .002-.003” seated too deep.

I am measuring these using a comparator and measuring cbto. My stem is not touching the meplat.

Who do some of these rounds seat deeper than others?

I use the same press, and when I fl size, they are all dead nuts on.

What do you consider acceptable tolerance for seating depth?
 
You're there, well within tolerance IMO...,. I prime, mandrel and seat bullets for my P.R. ammo on a Dillon 1050 and can have up to +\-.005" variation.

I can't see any practical difference on performance and the reduction in time is significant!
 
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Running a rcbs matchmaster micrometer seating die (223). My load is right on the verge of being compressed. You can hear a kernel or two of powder when seating.

Out of my last batch of 100 rounds , about 70 rounds are right at my desired seating depth. Another 20 rounds or so, at within .001”, and then I got a handful that are .002-.003” seated too deep.

I am measuring these using a comparator and measuring cbto. My stem is not touching the meplat.

Who do some of these rounds seat deeper than others?

I use the same press, and when I fl size, they are all dead nuts on.

What do you consider acceptable tolerance for seating depth?

Neck tension, friction, powder column height, anything that resists the bullet being seated.

Also, bullet variance. They not perfect.

A good load will be tolerant of plus minus what you’re seeing.
 
Those are good tolerances... best to ask yourself, are you seeing different results with those bullets seated differently? If not, not an issue
 
Measure the bullets and you'll easily see the up to 0.003" difference in bullet base to ogive variance for the ELD-M.
I was going to say this exactly. When you stack the headspace and bullet tolerances that's well within acceptable from the press/dies.
 
I was going to say this exactly. When you stack the headspace and bullet tolerances that's well within acceptable from the press/dies.
Maybe I’m missing something, but how would bullet base to ogive make a difference when measuring the final rounds from give to case base?

Wouldn’t it make more sense if there was a bullet variation from where the seating stem contacts the bullets to the ogive?
 
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Running a rcbs matchmaster micrometer seating die (223). My load is right on the verge of being compressed. You can hear a kernel or two of powder when seating.

Out of my last batch of 100 rounds , about 70 rounds are right at my desired seating depth. Another 20 rounds or so, at within .001”, and then I got a handful that are .002-.003” seated too deep.

I am measuring these using a comparator and measuring cbto. My stem is not touching the meplat.

Who do some of these rounds seat deeper than others?

I use the same press, and when I fl size, they are all dead nuts on.

What do you consider acceptable tolerance for seating depth?
When we talk about "seating depth", we're talking about how for the base of the bullet goes into the case and if you could measure that, it'd be the distance from the base of the case to the base of the bullet. Right?

Therefore, I often found the issue to be the difference between where the comparator measures the BTO (contact point of the comparator) to where the contact point of the seating stem. To get real consistent "seating depths", I sort my bullets based on the measurement from the base of the bullet to the seating stem contact point using a comparator that touches the same contact point as my seating stem. When filling cases to where they're being compressed, getting that bullet base to the same place would be important, IMHO.

For some brands of bullets, there's not much variance between the normal comparator contact point and the seating stem contact point. For others, there can be a significant difference. And in either case, there can be a significant difference between lots.

As far as acceptable variance in seating depth, I don't like to see more than .001". Since it's not unusual to use .003" increments in seating depth as one is tuning a load, I wouldn't want that much variance in my seating depth, though just .003 doesn't necessarily make a significant difference.

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Maybe I’m missing something, but how would bullet base to ogive make a difference when measuring the final rounds from give to case base?

Wouldn’t it make more sense if there was a bullet variation from where the seating stem contacts the bullets to the ogive?
That's essentially what is happening. It's faster and easier to use the bullet comparator to measure bullet base to ogive, but if you wanted to see the variation on ogive position relative to your seating stem. Disassemble the seating die, remove stem and use it as a comparator to measure the ogive position.

If you take a look at your bullet comparator insert, you'll probably notice the bore is basically a straight cylinder/concentric circle whereas your seating stem likely has a taper, radius, or angled geometry to better support the bullet during the seating process. This geometry on the seating stem and the profile of the ogive and its relative position is what changes the contact point of your seating stem on the ogive and gives you the variation.

Edit - Use bullet ogive variation as your baseline. Example - If you have 0.003" variation in bullet ogive position, but you have 0.005" variation in CBTO, something in your loading process contributed the extra 0.002".
 
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That's essentially what is happening. It's faster and easier to use the bullet comparator to measure bullet base to ogive, but if you wanted to see the variation on ogive position relative to your seating stem. Disassemble the seating die, remove stem measure the ogive position.

If you take a look at your bullet comparator insert, you'll probably notice the bore is basically a straight cylinder whereas your seating stem likely has a taper, radius, or angled geometry to better support the bullet during the seating process. This geometry on the seating stem and the profile of the ogive is what changes the contact point of your seating stem on the ogive.
So I take it that these seating discrepancies are actually due to bullets that are not completely uniform to each other…. essentially in the region of the ogive to where the seating stem contacts the bullet.

With that said, aside from manually adjusting my micrometer seating die for every single round…. There’s not a whole lot you can do?
 
One other thing you can do if you know you have some compression that may be causing small variations in your seating depths is use a drop tube to better settle the powder in the case.
Other than that your depth measurements look pretty consistent.
 
So I take it that these seating discrepancies are actually due to bullets that are not completely uniform to each other…. essentially in the region of the ogive to where the seating stem contacts the bullet.

With that said, aside from manually adjusting my micrometer seating die for every single round…. There’s not a whole lot you can do?
Yup. The realities of bullet manufacturing is that there will be variations. Some companies allow for higher variation and others have a narrower tolerance range.

If you're achieving the results you want on target in your shooting discipline and you don't require anything extra, then it's a matter of establishing that amount of variation (and the results on target) as the acceptable baseline. If you want the extra whether it's out of necessity or curiosity, then it's usually a matter of spending the time or spending the money, or both.

Adjusting the die is one option. Option 2 is bullet sorting. Both require extra time.
 
Running a rcbs matchmaster micrometer seating die (223). My load is right on the verge of being compressed. You can hear a kernel or two of powder when seating.

Out of my last batch of 100 rounds , about 70 rounds are right at my desired seating depth. Another 20 rounds or so, at within .001”, and then I got a handful that are .002-.003” seated too deep.

I am measuring these using a comparator and measuring cbto. My stem is not touching the meplat.

Who do some of these rounds seat deeper than others?

I use the same press, and when I fl size, they are all dead nuts on.

What do you consider acceptable tolerance for seating depth?
Let your target tell you. If you have a picky-ass bitch load that requires .005" accuracy you probably don't want that load or barrel. If you're a Hornady fan, none of that stuff is real and seating depth doesn't matter.

I do seating depth tests in 10 thou increments. If the load can't handle that, fuck em.
 
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Hornady 75eldm
That's the reason. 143 ELD-X have the same variation in seating depth show up, as well as the 75 AMAX. I actually noticed up to 0.020" variation in bullet base to ogive measurements when loading a new lot of bullets with the 75 AMAX. That same variation showed up when measuring after seating. For the record, berger have a almost no variation in those measurements, and lapua scenar were a very close second (even though they were supposedly less consistent than the scenar L). Once I'm done with my AMAX bullets I'll be going to bergers for my .223. Will also stick to 139 scenars after I'm through all my 143 ELD-X.
 
For those who fuss with minor OAL and think seating depth really matters ....

What's your measured throat erosion rate? Most of mine average between .003"-.005" per 100 rds. Suppose you shoot a 200 round match, what happens to your load?

I used to chase the lands as the throat eroded..... now I just load them all .020" off and shoot the barrel until the velocity drops.
 
For those who fuss with minor OAL and think seating depth really matters ....

What's your measured throat erosion rate? Most of mine average between .003"-.005" per 100 rds. Suppose you shoot a 200 round match, what happens to your load?

I used to chase the lands as the throat eroded..... now I just load them all .020" off and shoot the barrel until the velocity drops.
My gunsmith and I went on a binge measuring throat erosion for a bunch of different barrels. He does most of the local clubs barrels so guys would stop by after the Saturday match and he would measure their lands. From 6BR, to Creedmoors, to 6.5's they ran about 3-6 thou per hundred rounds.

What you're getting at is the two different theories on seating depth. One camp thinks the jump to the lands has to be maintained bc that's where their accuracy is. The other thinks that one seating depth effects barrel harmonics in a accurate way and you don't chase lands, you keep it there. As far as saying seating depth doesn't matter, set at 20 thou and never look back .... I think that's bullet dependent. And that's where disagreements come in. Because that is likely true for a bunch of quality bullets. 20 thou is a known good place to start and most good bullets will shoot well there. So I could see a lot people making a statement like yours and truly believing it...but not all bullets shoot best there. And there will be some outliers that don't agree with that and they're probably right. The 85.5gr Hybrid in my current 22GT barrel is the first bullet I've come across that shoots well at any oal. It just won't shoot poorly in this barrel. I have it at a 60 thou jump for no particular reason and I will probably leave it there for the life of the barrel
 
One of the more thorough, statistically reasonable tests that have been done on the subject is here:


Its a multi part series and it is bullet specific but I does a good job showing what can happen with seating depth and its interaction with throat erosion.
 
For those who fuss with minor OAL and think seating depth really matters ....

What's your measured throat erosion rate? Most of mine average between .003"-.005" per 100 rds. Suppose you shoot a 200 round match, what happens to your load?

I used to chase the lands as the throat eroded..... now I just load them all .020" off and shoot the barrel until the velocity drops.
I agree with what you're saying about the issue for throat erosion and it's effect on performance. But. . . seating depth isn't really about how far one is form the lands. Seating depth is about the case volume the powder sits in and how long it take for the bearing surface to pass by the case mouth rim for its contribution to blow by.

To your point, I did an experiment with my .308 and let the throat erode by .033", which took ~2,000 rounds before I say my groups move from being less than .5" to just over .5". That convinced me to not chase the lands. But to find a good load, I still need to find the right seating depth to get the best results on paper. Once that depth is found, it stays there until I see a significant change on paper, and that has little to nothing to do with the jump to the lands.
 
One of the more thorough, statistically reasonable tests that have been done on the subject is here:


Its a multi part series and it is bullet specific but I does a good job showing what can happen with seating depth and its interaction with throat erosion.
When that article was posted there, I was a little disappointed that he referenced "jump" when he was really talking about changes in "seating depth". While there's a relationship with jump and seating depth in terms of distance, in terms of their relationship for internal ballistics and on paper, it's separate and very different. IMHO, as good as he usually does on that website, that article gives the reader the wrong impression as to what's important and when.
 
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When that article was posted there, I was a little disappointed that he referenced "jump" when he was really taking about changes in "seating depth". While there's a relationship with jump and seating depth in terms of distance, in terms of their relationship for internal ballistics and on paper, it's separate and very different. IMHO, as good as he usually does on that website, that article gives the reader the wrong impression as to what's important and when.
I see your point and I agree with you. But I have a suspension that more people see it from the jump aspect as opposed to the other end of the bullet.
 
I agree with what you're saying about the issue for throat erosion and it's effect on performance. But. . . seating depth isn't really about how far one is form the lands. Seating depth is about the case volume the powder sits in and how long it take for the bearing surface to pass by the case mouth rim for its contribution to blow by.
Why do you think it isn't about both? Why do you think seating depth is only about it's affect on case capacity?
 
Why do you think it isn't about both? Why do you think seating depth is only about it's affect on case capacity?
I don't think it's "only" about its affect on case capacity (note, I mentioned an issue regarding bearing surface). The main point is that jump is not the overriding factor that should be focused on for load development. It if was, then 'chasing the lands" as the throat erodes would be an important thing to do, and it just isn't when jumping bullets. The focus should be on seating depth.

If one is talking about loading to touch or jamming the bullet, that's a whole different approach than for jumping.
 
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I don't think it's "only" about its affect on case capacity (note, I mentioned an issue regarding bearing surface). The main point is that jump is not the overriding factor that should be focused on for load development. It if was, then 'chasing the lands" as the throat erodes would be an important thing to do, and it just isn't when jumping bullets. The focus should be on seating depth.

If one is talking about loading to touch or jamming the bullet, that's a whole different approach than for jumping.
So your theory is based on experience that chasing the lands doesn't do anything and accuracy as it pertains to the length/depth/ whatever of seating a bullet comes from....what? Case capacity/ powder column something or barrel harmonics or what?
 
So your theory is based on experience that chasing the lands doesn't do anything and accuracy as it pertains to the length/depth/ whatever of seating a bullet comes from....what? Case capacity/ powder column something or barrel harmonics or what?
There was a time when I was "chasing the lands" to maintain my good results on paper. I measured and tracked my throat erosion and would adjust my seating to keep the same amount of jump. It frustrated me as it wasn't working and tried to figure out why. After some time and reading what Speedy does as what Cortina said about "chasing the lands", I decided to do an experiment. I found a load for my .308 that did really well and simply kept it there (absolutely no changes) for ~2,000 rounds with the throat eroding .033"-.034". During that time, the performance remained good and consistent. So, I was keeping my seating depth consistent, but the distance to the lands was constantly changing. That convinced my that seating depth was way more important than the distance from the lands. The only time distance to the lands is important is when starting a new load (like with a different bullet) to find a starting point, and that's it. From there, I don't pay any attention to distance to the lands for load development.

After that 2,000 rounds, I did find I had to make a small adjustment in my seating depth to get my results on paper back to where it was before. Changes in distance to the lands does have some effect, but not near as much as seating depth. And I'll point out too there's the issue with tangent ogives vs secant ogives and how they might interact with different chamber ramp leads.

So, yes . . . my "theory" comes from my testing experience, tracking the details and looking at the results. And my results align with what those experts were saying.
 
There was a time when I was "chasing the lands" to maintain my good results on paper. I measured and tracked my throat erosion and would adjust my seating to keep the same amount of jump. It frustrated me as it wasn't working and tried to figure out why. After some time and reading what Speedy does as what Cortina said about "chasing the lands", I decided to do an experiment. I found a load for my .308 that did really well and simply kept it there (absolutely no changes) for ~2,000 rounds with the throat eroding .033"-.034". During that time, the performance remained good and consistent. So, I was keeping my seating depth consistent, but the distance to the lands was constantly changing. That convinced my that seating depth was way more important than the distance from the lands. The only time distance to the lands is important is when starting a new load (like with a different bullet) to find a starting point, and that's it. From there, I don't pay any attention to distance to the lands for load development.

After that 2,000 rounds, I did find I had to make a small adjustment in my seating depth to get my results on paper back to where it was before. Changes in distance to the lands does have some effect, but not near as much as seating depth. And I'll point out too there's the issue with tangent ogives vs secant ogives and how they might interact with different chamber ramp leads.

So, yes . . . my "theory" comes from my testing experience, tracking the details and looking at the results. And my results align with what those experts were saying.
Okay, so if it's not jump to the lands what is the interaction that produces accuracy? You have said what it is not. What is it?
 
Okay, so if it's not jump to the lands what is the interaction that produces accuracy? You have said what it is not. What is it?
In terms of internal ballistics and for a very basic understanding: Seating depth, powder charge (barrel timing for bullet exit), consistent ignition.

A good barrel and receiver helps a lot too. ;)

Internal ballistics is very complex with many factors involved. Like, just look at what Speedy does for shrouds on this podcast (1:57:20):

 
In terms of internal ballistics and for a very basic understanding: Seating depth, powder charge (barrel timing for bullet exit), consistent ignition.

A good barrel and receiver helps a lot too. ;)

Internal ballistics is very complex with many factors involved. Like, just look at what Speedy does for shrouds on this podcast (1:57:20):


Okay. I don't think you have any special sauce. You were talking earlier like you knew something more than the rest of us so I thought I would try to draw out of you whatever great secret you we're hunting at, but it doesn't look like there's anything there.
 
Hornady 75eldm
I had suspected as much. I have this issue with the 178's in my 308 and the 150 eldx I've loaded in 280AI. And like an idiot, I bought 300 of the 208 eldm for my 300 winmag. I really only buy them because they're cheap and usually locally available.
 
Okay, so if it's not jump to the lands what is the interaction that produces accuracy? You have said what it is not. What is it?
When the barrel lets go of the end of the bullets bearing surface is the other side of the when the bullet touches the lands theory