OAL?

[Prof Young]

Loaders:

Was reloading 380 acp yesterday. Max length, according to the info on the die set is 0.984 inches. I seat the first bullet and measure it at 0.945. After all these years or reloading it occurs to me that there may be an optimum or recommended OAL. And yes, all my load data sources have a recommended OAL column. The 380, according to the Lee reloader manual, minimum OAL is 0.970. So I do the adjustment and get one at 0.970. I visually compare the 0.945 and the 0.970. One can't tell the difference it is so small. But I imagine that ballistically it does make a difference. A more deeply seated bullet is going to take more pressure to move.

Anyway, I'm wondering if, in a bullet this small, a bullet seated 0.035 too deeply really makes much difference?

I'll pay more attention to the minimum OAL for everything I reload now.

Talk to me.

Life is good.
Prof Young

[44 AMP]

A more deeply seated bullet is going to take more pressure to move.

Insignificant.

The pressure difference needed to move a deeply seated bullet is more than overcome by the powder charge. Enough more that the bullet is shoved out of the barrel at several hundred feet per second.

The issue, when there is one, is the result of the bullet being seated deeper, changing (reducing) the volume of the space inside the case. Loads are calculated and created so that X amount of Z powder fills Y space to a given pressure when it burns.

Change that amount of space without changing the powder charge and the pressure goes up. Whether or not this is a problem depends on many individual factors.

Max/min overall loaded lengths are set based on the function of the rounds with standard bullets in standard guns. Max is set to not shove the bullet into the rifling, and to work through the actions & magazine. Minimum length is not a fixed value but a general length that will work in most guns.
RECOMMENDED minimum length is just that, a recommendation.

SAAMI specs are for people making and selling ammo as a commercial enterprise and are set so "everyone" is working to a common standard. They are NOT the physical safety boundaries.

[L. Boscoe]

some COL will not feed in a semi auto, but other than that, I find
no difference in fps with the same powder load in 38 super with
more than 0.050 difference in the recommended 1.260, even
.080 less works and does not affect fps.

[Nathan]

Im confused by your logic here. Typically, I find some combination of plunk test or comparison to factory ammo the gun likes to be the way to determine pistol OAL.

https://www.youtube.com/shorts/4qRgKOCo93I

[stagpanther]

Your barrel's chamber is the best case gauge--the only thing I'd add is it's also a good time to check support of the case head.

[44 AMP]

Typically, I find some combination of plunk test or comparison to factory ammo the gun likes to be the way to determine pistol OAL.

Your barrel's chamber is the best case gauge--the only thing I'd add is it's also a good time to check support of the case head.

If you're feeding a semi auto pistol, there is one more thing you need to check. The Magazine. You need to ensure that the rounds feel not just from the magazine to the chamber, but all the way from the bottom of the magazine to the chamber.

Its not common, but its not impossible to create a combination of bullet length and seating that will chamber, and feed the top, or top two rounds from the mag but will bind up deep in the magazine body.

Pretty rare but I've seen it happen.

[stagpanther]

You need to ensure that the rounds feel not just from the magazine to the chamber, but all the way from the bottom of the magazine to the chamber.

Its not common, but its not impossible to create a combination of bullet length and seating that will chamber, and feed the top, or top two rounds from the mag but will bind up deep in the magazine body.

Pretty rare but I've seen it happen.

Which is why I avoid anything that is fed by Glock magazines--except Glocks where you have no choice.

[Metal god]

The smaller the case and faster the powder , the more seating deeper effects pressure . So I’d say yes it makes a big difference compared to say seating a 30-06 .025 deeper .

In fact, that is my seating progression when adjusting seating depth to fine tune my 308 loads to the point of the bullet being moved as deep as .125 deeper from where I started but I would never fine tune my 9mm loads like that .

I guess the question would be what is the difference you were asking about . Is it gonna make a difference on paper? Is it gonna blow up your gun ? Is it gonna increase velocity , , could it be the difference between feeding and jamming all the time ? I think it could make a difference in all of those things. Some of them won’t be noticed others might.

[nuchie11]

In something like .380 ACP, that small 0.035” difference can absolutely matter. You’re dealing with limited case volume, so seating a bullet deeper increases pressure pretty quickly. That might not be catastrophic with mild loads, but if you’re near max charges or switching powders, it could push pressures into unsafe territory. Even if it’s not dangerous, it can impact reliability, cycling, and accuracy.

[jetinteriorguy]

I do my seating depth determination before I even start loading. I start by seating a dummy round at max length that fits all my different pistol mags and is at or under SAAMI max. I’ve actually determined this over the years so I know this starting point. I then take the barrel from my CZ, which is the most sensitive to OAL and keep seating until it plunks then add another .010 to allow for minor discrepancies. This is the point I start from when working up a load.

[Jim Watson]

The Speer manual gets real stern on 9mm OAL being no less than shown in the data.
There was once a story that seating a 9mm bullet even .060" (1/16"!) deep would increase chamber pressure a lot.

On the other hand, Vihtavuori once showed the results of seating a .308 bullet progressively deeper. At first, chamber pressure declined due to the greater bullet jump acting as freebore. Only until it was a good bit deeper did reduced powder space cause pressure to go back up.

Phil Sharpe tabulated loads by bullet seating depth, not OAL. Called for some arithmetic but gave a better handle on powder space effects.

[Prof Young]

Thanks for all the information. I need to digest all this and see how it applies to what I load. I can see how minor variations may not matter, but that there are a lot of variables in play.

I think standard procedure for me, from this point forward, will be to not exceed the max recommenced OAL, and not be less than the minimum.

Lots to learn.

Life is good
Prof Young

[Unclenick]

If I seat a 150 grain bullet 0.060" deeper in a 308, it eats up about 2% of the total case volume. If I seat a 115 grain bullet 0.060" deeper in a 9×19, it eats up about 14% of the space. So the additional powder confinement is about 7 times greater in the 9×19.

If you run the numbers in an interior ballistics calculator, they both show pressure increase, but for a given increase it's a much greater percentage in the 9×19.

But now we get a reality check. It won't always happen. Hartmut Broemel points out that there are times when a magnum primer reduces pressure and velocity. What is happening, for pistols, in particular, is the primer unseats the bullet and starts it forward before the powder gets burning up to speed, so you don't actually shoot it from the seating depth it was loaded to. So in that instance you don't know, without measuring, what your pressure is doing.

In rifles, it gets messier. If you seat too deeply, pressure goes up. But if you seat too far out near the lands, the pressure goes up, too. Dr. Lloyd Brownell showed that rifle pressure (he tested 30-06) is affected by gas bypass, wherein, before the bullet moves appreciably, the case neck expands outward against the chamber and starts leaking out of the mouth past the bullet. In these situations, pressure drops as you start backing away from contact with the throat, then reaches a bottom value after which seating still deeper starts raising the pressure again as the influence of reduced space starts to take over being the dominant factor from the throat proximity.

[Metal god]

FWIW and if anyone cares haha
As it relates to my 308 rifle loads . I almost always seat .020 off the lands to start and why I go deeper to adjust . In my new barrel it usually has most of my COAL at and often over 2.900 with one heavy match bullet being 3.0 something coal . Meaning I usually have lots of room to go deeper .

[BJung]

Here are photos of printed bullets that support my experience.

Firstly, I discovered that when the COL increased, I needed more powder to maintain accuracy. Here is a classic 38-148WC load..

Secondly, I think a COL matters. Here is a comparison of a 45-225 RN load with different COL. Hmm. I cannot download the picture. Anyway, the longer bullet was more accurate than the cartridge with the shorter COL

Third, I loaded 40-170TC testloads. I don't have a photo but the printed group of the longest COL was larger than a shorter COL. This time, I admit, I used my powder measure than trickling up to a weight. I will retest. Yet, if correct, The longer COL doesn't guarantee the best accuracy. The gun has to be able to cycle the load too.

[Unclenick]

With cast bullets in the 45 Auto, I always seat them out to contact the throat. It improves accuracy substantially (I've had it shrink groups as much as 40%). With jacketed bullets, I can't tell any difference.

[Jim Watson]

I have been "headspacing" on the shoulder of a .45 SWC since a club member started supplying me with them back in the previous century.
I went back to roundnose when I for some reason lost the best feeding. I wonder if the mold dimensions shifted as they made new cherries.

[stagpanther]

With cast bullets in the 45 Auto, I always seat them out to contact the throat. It improves accuracy substantially (I've had it shrink groups as much as 40%). With jacketed bullets, I can't tell any difference.

Is it possible that is more of a function of "form fitting" (sorta like 22lr) the lead bullet to the throat/lead/rifling than it is strictly a question of "no jump?"

[Unclenick]

Based on the lead fouling that would build up as a ring next to the throat when I didn't do it, I believe the bullet is shaving itself off against the step that forms the chamber's mirror to the case mouth; i.e., the headspace determinant. When I do it, I seat so that the back end of the case is flush with the back end of the barrel extension on the 1911. So, the bullet is actually aligning the whole cartridge axis with the bore axis. A bullet that is shaved on one side is out of balance and wobbles in flight after it clears the muzzle.

I found long seating like that to be ot no help with jacketed bullets. Apparently, they are strong enough to center themselves in the throat under pressure without deforming.