Bullet seating depth and load development

[Tom68]

I’m curious about the role in seating depth and whether it is an independent factor in load development (along with powder choice, charge weight, bullet construction, primer, brass, etc).

If I begin load development with bullet “X”, trying a variety of combinations of the other factors, and then identify the “best” load of those that I have tried… would varying seating depth throw that combination into disarray? Or, put another way, would trying a different COAL possibly produce a better result in one of the previously tried—but discarded—powder/charge/etc combinations?

I realize that changing bullets will often require re-evaluating seating depth, so for sake of discussion, I'm considering only one bullet at a time. I'm thinking that it is NOT an independent factor, and that tweaking the seating depth is part of fine-tuning what appears to be the best combination of the other components. I THINK that the best approach is to uniformly seat bullets across the OCW spectrum... and then try varying the seating depth for the best combo. Looking for input... or to change my mind.

[black mamba]

When working up new loads I start with COL at maximum, with the bullet as close to the rifling as possible without touching. With many rifles it is not possible to get close to the rifling because of magazine box length, but I always start load workup as close as I can get. The reason for this is that seating deeper in the case (farther from the rifling) actually decreases pressure, so it is very safe to do. If you work up to a max load with the bullet 0.030" from the rifling, and then want to try 0.015" from the rifling, the pressure could jump too high and be hard on the rifle or even dangerous.

The exception to this rule is in using monolithic bullets, such as Barnes X or Hornady GMX. These harder bullets need a better "run" into the rifling, so it is recommended that they be seated farther from the rifling, around 0.050".

This process is for bottleneck rifle cases, not for straight-walled pistol cartridges. With straight cases, the deeper you seat the bullet, the pressure increases due to a smaller powder space.

[JefferS]

Read through this thread. Great info here.
http://thefiringline.com/forums/showthread.php?t=535919

[Sevens]

This process is for bottleneck rifle cases, not for straight-walled pistol cartridges. With straight cases, the deeper you seat the bullet, the pressure increases due to a smaller powder space.

Before the thread develops further, I'll quote this bit. The OP did not state specifically if we're talking bottle neck rifle rounds or handgun ammo. His use of the OCW method pretty much tells me that he's talking rifle ammo, but it's important we make the clear distinction.

In rifle handloading, I will defer to sharper minds on the subject.

In handgun handloading, COAL is a completely different animal and at least with regards to safety, it's extremely important to have an idea of what you're doing and how you go forward.

[Tom68]

Sorry, should have stated this up front: yes, I am speaking specifically about bottleneck rifle cartridge loading.

[flashhole]

I use the bullet manufacturers COL when performing ladder tests to determine an Optimum Charge Weight per Don Newberry's method. I view seating depth as a fine tuning to accuracy and this method has worked well for me.

[Bart B.]

I don't think there's much difference in accuracy between bullets seated deep enough to easily go in and out of magazines and those seated out to touch the lands or even be set back a few thousandths when chambered; providing they're pretty straight on the case shoulder to start with.

Sierra's bullets tend to shoot most accurate when gently pushed into the lands. Berger's VLD bullets tend to do best a few to several thousandths less. The angle of the leade in the chamber where the rifling starts plays into this issue, too.

For ammo to be used in magazines, load to an OAL just long enough to easily work with the magazine.

For single round loading and shooting, I suggest starting with them touching the lands, then back off in .005" steps. Shoot at least 10 shots per test group; 20 if you want 90% plus reliable data. Use whatever you think is best.

[Gary L. Griffiths]

FWIW, I experimented with some SS-109 (62-gr FMJBT) bullets with identical cases, primers, and loads, one batch seated to normal depth, and one seated deeper to simulate the seating depth of Barnes 62-gr TTSX bullets. Neither gave any signs of excess pressure. The normally-seated bullets averaged 21 fps. faster than the deep-seated ones, however the deep-seated ones were slightly more consistent, with a standard deviation of 20 fps. vs. 29 fps. for the normal loads.

[Brian Pfleuger]

It is well known that seating depth has an effect on accuracy that is relatively independent of powder charge. That is, changes in OAL while maintaining the same powder charge can dramatically effect accuracy. This is particularly but not exclusively true of VLD (secant ogive) bullets.

It is also well known that powder charge changes can have an effect on accuracy that is relatively independent of OAL variations.

Changes in powder charge changes many variables, as the amplitude of vibrations increase with increased powder/pressure and these vibrations happen in 3 dimensions and in multiple forms in each dimension. There is also a significant change in barrel time and angular forces on the bullet, just to name a couple other factors, when changing powder charges.

Changes in OAL when using high pressure loads also change those same variables (since changing OAL also changes pressure) plus more variables of it's own.

When examining the effects of changing a variable in any system it is highly desired that the variable in question be isolated. That is, no other (or as few as possible other) variables are also changed, else the variable having the observed effect may be obscured.

So, I have this theory that I wish I had time to test before now. I have a batch of ammo made up but I haven't had time to shoot it.

Small charges of very fast powders will produce low pressures and small vibrations that dampen quickly, because the powder is done burning sometimes before the bullet clears the case mouth. These same charges exhibit relatively insignificant variations in pressure and thus velocity with changes in OAL.

Hence, with such a powder, a shooter can experiment with OAL without having significant effects on the other variables.

That powder is IMR Trail Boss. 100% case fill loads produce max pressures typically around 50% of the cartridges max PSI in modern, high-pressure bottle-neck cartridges.

The process would look like a modified version of Dan Newberry's OCW wherein the standard variable of powder charge is replaced with OAL variation and the powder charge is kept constant.

Hopefully, this technique will, as much as possible, isolate the OAL variable and allow the shooter to find the preferred OAL before using the standard OCW technique to find the optimal powder charge.

[Bart B.]

I have problems accepting Newberry's OCW theory. It seems to be based on the premis that bullets exiting when the barrel's straight out shoot most accurate. Note that barrel time for centerfire cartridges is typically 1.0 to 1.3 milliseconds; .0010 to .0013 seconds. And the harmonic frequencies of the barrel are not important, just the main or resonant one.

Calculating the resonant/fundamental frequencies of several 30 caliber barrels from 2 pound skinny 22" featherweights to 6.5 pound 28" long thick match barrels, they range from 39 to 71 Hz (Hertz or cycles per second). The time for each to go from straight out to straight out in one-half cycle ranges from about 7 to 13 milliseconds. The bullet's long gone before the muzzle whip goes through one-half cycle and points straight out again. And that high-speed shock wave Newberry talks about makes 4 to 5 cycles, back to front then back, while the bullet's going through the barrel.

[hooligan1]

First of all, I agree with a little of all this informatio and the link in post #3, as they all have valid steps in producing accurate and consistant ammunition.

I have use Dan Newberry's OCW method to some extent and have come to the conclusion that his method can and will produce great results.

I also use the coal adjustment method explained in the link posted by JeffrS.

I now use this method only I try to start with the minimum powder charge and stay with this charge until a consistantly accurate BTO (base to ojive) measuresurement is obtained from a given bullet, and then I work up powder charge until group opens and pressure signs begin to appear... This method helped me find what's now one of my best loads for my Model 70, 7mm rem mag, with Berger's 168 VLD hunting bullet using IMR 7828, with Minimal component loss due to testing.
The only change to the BTO method was I only used 4 rounds, and it only took three different measurements until I was happy with the bto, then I upped the powder til grouping became tighter and more consistant with more velocity...

I say more velocity, I havent chronoed this new load yet and no matter what it is the rifle loves it so much I zeroed scope to this load at 100, and once I figure out howbto post pics using my junky phone I will start a thread on this very load developement, maybe it will help loaders knock off some wasted effort in load developement.

[WESHOOT2]

tweaking the seating depth is part of fine-tuning what appears to be the best combination of the other components

And primer; I final-tweak OAL and primer (primer first ).

[green_MTman]

I start at .010 off the rifling then back off if pressure is to high or accuracy is not good.

With copper bullets as said previously,,you need to back off the grooves to at least .050 or .075 as said

[emcon5]

Perfect is the enemy of good.

I do the "sliding bullet in a case" method. Size a case, then cut a slot in the neck, so there is enough tension to grip the bullet, but not so much tension that it will push the bullet into the rifling and stick.

Then chamber the round. Measure the OAL, and set your seating die to match that OAL, and then adjust from there. What has worked for me is to simply take that value and give the seating stem in the die a 1/4 turn. That gives it a uniform jump to the rifling (about .015" on Lee dies), close enough for good accuracy, but not so close that any bullets get stuck if you try and unload.

Am I leaving any accuracy on the table? Almost certainly, but using good components, match primers, and a ladder test to get a load, followed by a 10 round group to confirm, I have so far always come up with a load that meets my accuracy needs.

I like to shoot, and reloading is secondary. I could easily shoot another 100 rounds chasing a few tenths, but that is not my idea of fun. I know for some people it is, and more power too them.

[WESHOOT2]

Best accuracy is not always found with the bullet closest to the lands; sometimes it's found at short(er) OAL.

[Bart B.]

When you make ammo for a new barrel establishing some distance from the rifling to seat bullets at, do you seat bullets out further as the rifling wears away?

I ask because all the .308 Win barrels I wore out had their leade advance about .010" for every 300 rounds fired. A 30 caliber throat erosion gauge went in about .100" past the new barrel point for every 600 rounds fired.

My 30 caliber magnum barrels' leade advanced over twice that rate. A .264 Win Mag barrel wore out about five times as fast; 640 rounds total.

[emcon5]

When you make ammo for a new barrel establishing some distance from the rifling to seat bullets at, do you seat bullets out further as the rifling wears away?

I ask because all the .308 Win barrels I wore out had their leade advance about .010" for every 300 rounds fired. A 30 caliber throat erosion gauge went in about .100" past the new barrel point for every 600 rounds fired.

Yes, on my 6.5-06 I am nearing the point that I can't do it anymore, as the leade is simply getting too far away. Accuracy is still tolerable, so I'll have to see how long that lasts, but I am not optimistic.

I think some new plumbing is in my future.