40SW case variation

[hubcap500]

I did a random weight check on my 40SW brass (assorted headstamps). It was cleaned and unprimed--just the brass. I checked the case length and they were all the same, but as much as 10.5 grains weight difference!!! Is that too much? I don't usually sort handgun brass, but should I?

I'm not as concerned about the lack of weight uniformity. I doubt there are many, if any, people who could tell the difference in accuracy with a hand-held firearm at normal pistol distances with the kinds of deviations I'm talking about here. I'm not talking about bench-rest accuracy.

My concern is not the presence of the weight/capacity deviation, but the amount of deviation-- well over 10%. I don't know how much a % of change in case capacity affects pressures, only that it most certainly does. All reputable manuals advise reloaders of semi- or full-auto firearms to make sure the ammunition does not allow for bullet setback for this very reason.

My concern is about the safety of the ammo, because the weight is a factor in determining case capacity, and therefore pressure, and therefore safety.

If you change your powder charge by that amount, you can very quickly go from safe to unsafe (or vice versa) pressures. It's also my understanding that, because of the nature of smokeless propellants, the pressure curve can rise exponentially rather than linearly at unsuitable pressure levels (high or low).

 

[FrankenMauser]

Be safe, rather than sorry.
Sort at least by headstamp.

I do not recall ever having measured volume and compared it to case weight. But, that's probably because I use all cases as-fired, and they all come from different firearms (negating the effectiveness of checking volume).
But... I have seen weight correlated to volume.


I use .40 S&W cases as bullet jackets for swaged 44 caliber (0.429-.432") bullets; the majority of which see use in rifles.
As such, consistency and uniformity is very important.
I sort by head stamp, and then by weight.

After being fully processed, the average case weighs 72 grains (73 gr for Winchester - my preferred jacket).
Those that fall short of the average, after swaging, will have the lead core sunken farther down inside the jacket; while cases above the average will have the lead core flush with the jacket, or even above. ("Average" bullets have the lead core slightly below flush with the mouth of the jacket. -- Sometimes called a "protected soft point" by commercial manufacturers.)

Based on that experience, I assume that case weight does correlate to volume ... whether it means thicker walls, a thicker web, and/or a stepped case wall.

Note the two bullets in the background.
Both Winchester. But different weights and different case wall thicknesses, from different production runs of brass:
(So, contrary to my original statement, even sorting by headstamp wouldn't help in that particular situation.)

These two are just for fun. I pulled this bullet out of a bull elk:

 

[Sevens]

FM, no! I will not ever get tired of seeing those pictures! Those pictures make me smile... so cool.

To the subject, I wouldn't worry unless you are already running or planning to run max loads or beyond. That's just me. For darn sure, just as you pointed out, I am cognizant (hyper cognizant!) of unnoticed and/unintended bullet setback, and bullet pull/case mouth tension is something that I am rigid and unrelenting about, but unless I were running full-nuts max loads, I wouldn't (myself) be concerned about that weight variance.

However, with that said...
It's not fair to so positively deduce the amount of space variance simply by weight alone. If you wish to get to the nitty gritty of it and truly see how much physical space variance you actually have, you need to measure it with water.

I believe it's more legwork than it is worth, but it will tell you more than simply weighing cases.

Perhaps more because I'm too lazy to do it but also because I truly don't need to (but am still curious...) I would love to hear what you find, to see how much of a direct translation weight of the entire case relates to combustion space.

This is an interesting discussion for sure, but I just don't see much worry about pressure issues myself.

 

[Kevin Rohrer]

You are overthinking this.

Clean, resize, and inspect them. If they are no longer than the max OAL (which they won't be) load, shoot and repeat until they split.