Conundrum time...

[daved20319]

So as most of us know, rimless semi-autos generally headspace on the case mouth.  In thinking about this, it then seems like case length and crimp would be critical if you're chasing best accuracy, and lowest SD and ES.  But it seems most folks say trimming pistol brass is a waste of time and/or makes no difference.  So how can that be  ?  Is there so much slop in the headspacing that .020" doesn't matter?  I say .020" as that's the typical range for case length.  And what about crimp?  Seems if you overdo it, you risk not having a case mouth to headspace on.  So what am I missing here, guys?  Thanks.

Dave, aka Confused in Twisp

[eastman]

My thought is that it only really matters if the case is too long. The case head is held by the extractor, so if the case mouth doesn't actually stop at the end of the chamber it doesn't affect lock up.

If the bullet is seated out too far and contacts the rifling, that is an issue whether or not the case mouth is bottomed out in the chamber. COAL is more important than case length.

Anyone with a different experience will certainly comment. I base some of this on my experience at a demo event/manufacturer shoot last year. There were complaints that my Scorpion pistol wasn't cycling. Turns out they were mixing up ammo and the 9x19mm ammo bowls had some 9x17mm mixed in. They fed and fired fine, but of course don't have the power to cycle the Scorpion bolt.

[George16]

I’m with Eastman. I still have to encounter a 9 x 19 case that’s too long. As such, I don't trim any of my pistol brass. However, I load using single headstamp brass to lessen the variation in OAL and this helps a lot in having los ES and SD. I also crimp the bullet just enough to remove the flare.

[Wobbly]

• It's my understanding that pistol brass gets shorter as it's repeatedly reloaded. Maybe you are confusing this with bottle neck rifle that needs trimming exactly because it does grow in length.

• And who said anything about 0.020" ?? All my brass falls within a length range of about 0.008".

 

[Earl Keese]

Maybe start taking measurements instead of making assumptions. I find this helps alleviate confusion.

[M1A4ME]

You shouldn't be crimping a 9MM case like you would a .38 or .357 anyway.

The bullet should be held in place by the friction between the sides of the bullet and the inside of the case/mouth area the bullet sides contact.

The "crimp" is to remove the bell/flare your dies put on the case mouth to aid in bullet seating.  The case/cartridge won't chamber properly unless the bell/flare is removed.

Does the case mouth contact the end of the chamber?  Sure.  I hear it every time I do a plunk test.

If I really want best accuracy in rifle reloads I try to keep similar brass together.  I used to do that with pistol brass when I loaded less rounds.  A box of brass stayed together time after time after time.  These days I try to keep the same brand together except for the mixed stuff loaded for plinking.

[daved20319]

Thanks for the responses, guys.  Just to be clear, I'm not making assumptions or crimping like a revolver, this was one of those questions that popped into my head while planning my next loading session.  And bottle necked rifle rounds have nothing to do with it, I know they head space on the shoulder, not the case mouth.  As to the .020", most data I've seen has a case length range of .020", with the trim to length falling in the middle of that, or .010".  That's for rifles, of course, a recent check shows pistols at half that, guess there was an assumption there.

Anyway, it just seemed odd that for a cartridge that head spaces on the case mouth, case length wouldn't be more critical.  Thanks again.

Dave

[Wobbly]

Anyway, it just seemed odd that for a cartridge that head spaces on the case mouth, case length wouldn't be more critical.  Thanks again.

Dave -
It certainly IS more critical. That's why the tolerance range for pistol brass is 0.010" instead of 0.020" found with rifle. From those numbers, I'd have to say it's about 2 times more critical. So in that respect you are correct.

As it turns out for auto pistol, variations in powder (consistent powder weight across a group of cartridges) and variations in internal case volume have a much greater affect on SD and ES. This is why most of us will test with a single brand of brass, even if we use mixed brass in production.

Hope this helps. 

[daved20319]

Anyway, it just seemed odd that for a cartridge that head spaces on the case mouth, case length wouldn't be more critical.  Thanks again.

Dave -
It certainly IS more critical. That's why the tolerance range for pistol brass is 0.010" instead of 0.020" found with rifle. From those numbers, I'd have to say it's about 2 times more critical. So in that respect you are correct.

As it turns out for auto pistol, variations in powder (consistent powder weight across a group of cartridges) and variations in internal case volume have a much greater affect on SD and ES. This is why most of us will test with a single brand of brass, even if we use mixed brass in production.

Hope this helps. 

Thanks, Wobbly.  I guess it's just the machinist in me, .010" is a LOT in that world.  But hey, it's working, I guess that means I don't need to fix it  .  Later.

Dave