Pistol cartridge seating depth

[Fioleks4181]

I've been reloading for about a year now. Mostly I reload 40 s&w, and obtain brass from ranges and gun shows. I always measure each piece of brass before sizing, and then again after. Next I would trim them all down to recommended trim length. It seems very tedious and time consuming, especially since I'm using the hornady classic
Press. I have seen I others saying there is no need to trim pistol brass. So the main question is if you have multiple length cases, what effect does it have on seating depth when you set the seating die to COL?

 

[Howland]

Inconsistent case length will have absolutely no effect on OAL. Because the cartridge headspaces on the case mouth, failure to lock into battery can occur if case length is too long. It would have to be way too short to cause failure to fire.

Before it even gets into the pistol, different case lengths can cause trouble at the loading bench. Widely varying case length means longer cases will get an excessive bell and crimp, short cases not enough of either and possibly no crimp at all.

I've never loaded .40 S&W, but I've loaded tons of 10mm. I've always measured cases but only after sizing, and only trimmed those exceeding maximum and only trimmed those back 0.005". A lot of measuring but very little actual trimming.

 

[Gary Wells]

The only trimming you are doing is the life expectancy of the case & your billfold. Straight walled piston cases do not need trimming. They trim by themselves. Shouldered cases might be different. Clean, reload, inspect, & shoot.

 

[dahermit]

The only trimming you are doing is the life expectancy of the case & your billfold. Straight walled piston cases do not need trimming. They trim by themselves. Shouldered cases might be different. Clean, reload, inspect, & shoot.

It would seem to me that trimming them all to the same length is the proper way of dealing with cartridges that head space on the mouth. Nevertheless, once all my cases are uniform length, I don't agonize over case-length again. It must be my concern for precision.

 

[mikld]

I started reloading semi-auto rounds in '90, 45 ACP. I added 9mm in '95. I have reloaded these calibers for 5 guns and have not trimmed a case yet. Perhaps in theory the "consistent crimp" thinking is valid, but I do not "crimp" any semi-auto handloads, I just "deflare" with a taper crimp die. Headspacing is another idea that may be correct in theory, but in tens of thousands of rounds I have never had a misfire from cases too short. If one wants to trim 40 S&W brass, then fine, no harm no foul. Is trimming necessary? Highly doubtful...

I determine OAL by starting with max SAAMI length then reduce length until I get good plunk test results, or use the bullet manufacturer's recommended OAL and plunk test...

 

[T. O'Heir]

"...seating depth..." Is kind of a misnomer. You're loading for a specific COAL not a seating depth. Doesn't really apply to bottle necked rifle cases either, but can with those.
Trimming is more of a 'not likely to be required' thing than a 'not needed' thing. It's never an 'every time' thing(isn't with bottle necked rifle cases either). Most pistol cases don't stretch a great deal, if at all.
Anyway, as long as the case length is between the max case length and the trim-to length, both given in your manual, the cases are fine and you don't need to trim or worry about 'em. So you can check 'em but setting your vernier calipres to the max case length and using it as a case length gauge. Quick and easy.

 

[Gary Wells]

Quite a few .45 auto case manufacturers are already on the low limit to already undersize by a few thou. And guaranteed they will shorten, not lengthen during their shooting-reloading cycles.

 

[Unclenick]

if you have multiple length cases, what effect does it have on seating depth when you set the seating die to COL?

Welcome to the forum.

The direct answers to your question are that variable case length has no effect on COL as that is determined by the seating die's distance from your press shell holder and will be the same regardless of case length. Seating depth will vary inversely with case length as it is defined as how far below the case mouth the bullet base extends.

Seating Depth = Case Length + Bullet Length - COL

However, seating depth is not really a very useful measure if the case lengths are not the same. This is because what you are actually interested in getting from this measurement is how much powder space there is under the bullet. That changes with COL, but if you have the same COL on a short case and on a long case, the powder space under the bullet is the same. For that reason, a much more useful form of the Seating Depth calculation just pretends all cases are the same length.

Seating Depth = SAAMI Maximum Case Length + Bullet Length - COL

or you can use:

Seating Depth = Nominal Case Trim-to Length + Bullet Length - COL

Either of those will give you seating depth inversely proportional to the amount of powder space under the bullet. With same-weight, same-construction bullets of different nose length, adjusting the COL's to give the same seating depth by one of those two above formulas will let you use the same powder charge in both.

Getting your cases trimmed to the same length will make crimp pressure consitent, and that's the main reason people do it. Some taper crimp dies have a gradual enough taper that it isn't critical, but if you use a short roll-crimp or short taper crimp length die shoulder to just barely take the expander flare off the case mouth, you can get some variation is how effecting that is, being tighter on longer cases than on short ones.

If you shoot your 10 mm near maximum, you may find your cases stay pretty much the same length for as long as you own them, so you can trim once for uniform crimp and never need to do it again for the life of the cases.

If you shoot reduced target loads, you may actually find the cases getting a little shorter with each reloading cycle, so in that instance you want to trim them as long as the shortest case in your lot and try to load all the cases the same number of times with the same load so they all shorten at the same rate.