Brass length

[Peter]

I'm fairly new to loading for semi auto pistols. I just bought some Win. 10mm brass, after mic'ing about 30 or so i found they are from .885 to .896 and I was just wondering 1. What is the minimum  length you should trim them to and 2. Should I trim them? I just bought a Kimber Custom II. Like i said I haven't reloaded much in semi auto, I reload for single shot or single action pistols mostly although I do have a 22tcm (semi auto) that I load for and is a hoot and a half to shoot. Thanks for any help. Peter

[Kenk]

Hey Peter, not sure about the other folks, but have never had to trim my 10mm brass...which could have something to do with the fact that I lose most of what I shoot at my outdoor hey field range 😀

[llamaboy]

There are three reasons for trimming brass.

The first is that you are very anal, and can't help yourself.  Ha ha

Secondly, you don't want to exceed max case length.  In this case, 0.992".  I don't think your cases will last long enough for that to be a problem.

Last is uniformity.  It's been shown many times that gross discrepancies in case length results in poor groups.  The lone flier in an otherwise tight group is usually attributed to this and not having a square mouth, if no other cause can be found.  This applies to both rifles and handguns.

As far as min length goes, it depends on what you're going to use the pistol for.  If it's just general plinking and range fun, then I wouldn't bother trimming at all.  If you are in competition or hunting, then you need to trim and sort.  Take a sheet of lined paper, and enter the numbers .883 to .897 on every other line.  Measure the case lengths and put the case next to the corresponding number.  When you are finished, you should have a nice Bell Curve.  Take the mass of cases that are at or near the top of the bell, and trim them to the length of the shortest case in that group.  Let's say that out of 100 cases, you have 67 (2/3) of them in a bell from .891-.894.   Take these and trim them to .891.  Then chamfer the mouths, and weigh them.  Put any cases that fall out of the group with the rejects from the first step.  What you are left with are your competition/hunting cases.  They should all have pretty much the same capacity, and be very consistent.  That is all they are to be used for.  Not for practice and not for plinking. 

I'll tell you a little story why I do this.  A few years ago, I was down on the Big Hole River hunting elk.  Now this was the middle of November and a typical sunny day with a temp of about -20.  Elk and deer do not run around at this temperature.  They just walk.  Anyway, I'm going along and suddenly find myself right in front of a herd of bedded elk.  So they, one by one, get up and walk away.  All I am offered is the Texas heart shot.  After quite a few of them walk away, I'm getting a little anxious, so I try a head/neck shot on one.  As I'm reloading, I don't see any affect on the cow, but I do see another one stand up, broadside, about 30 yds away.  As I turn to make a sure thing shot, I can't close the bolt on my rifle.   I look down and see a spent primer laying where it makes the rifle inoperable.  Did I mention that it was a little cool out, and I'm wearing two pair of heavy gloves?  The primer would not shake out, and I couldn't dig it out with the gloves on.  By the time I got everything in operating order, the herd had walked away.  Where did the primer come from?  It was a loose primer that fell out at the shot.  Never again!  I have one box of ammo for each hunting gun that is labelled "HUNTING" and that is all they are used for.  I topped off a box of ammo for my daughter a couple of years ago.  She has gotten an antelope in each of the last 2 years.  There are 18 loaded rounds in the box.  I think I will die before that box needs to be refilled.  I am 76 yrs and 362 days old.

The cases that didn't make muster are the ones you use for practice.  And that is how that box is labelled.  If a primer falls out at the range, no big deal.  I, on the other hand, lost 300# of prime meat.

[Kenk]

Thanks so much llamaboy. At this point I only use factory Underwood ammo for whitetail so I should be safe on that one 😊

[fltbed]

1. What is the minimum length you should trim them to and
2. Should I trim them?
Thanks for any help. Peter

Hello Peter and welcome to the forum.

I'll answer your second question first.  No, you shouldn't trim.  Why you ask?
straight wall autoloading pistol cartridges (unlike rifle cartridges) grow shorter the more you shoot and reload them.  Why?  Their are a lot of theories on that and it makes for great debates around the coffee pot but suffice it to say, they're enough tests done that we know autoloading pistol cartridges actually headspace on the extractor and not the case mouth.

As for the minimum trim length?  That's almost impossible to say as almost every 10mm owner has or will attempt to shoot 40S&W out of their 10mm and discover...yes...it does work.  (I won't get into the details or possible dangers but suffice it to say that's another great subject for discussing around the coffee pot.)

Most of us that pursue accuracy, have done what llamaboy suggested, because I believe matching your case length to your chamber headspace improves accuracy.  (I personally use new Starline cases for such endeavors)   Some have even gone as far as cutting down other calibers to make brass fit their chambers better. (i.e. cutting down 451 Detonics Mag brass down to the length of a 45acp match chamber)

Anyway, I've rambled on long enough.
Hope this helps.

Jeff

[Kenk]

Great explanation, Thanks Jeff!

[Peter]

Thanks all. I to use Starline brass if I can but in the times we have right now I felt like I won the lottery when my local Scheel's got brass in. There is no chance of me ever using 40S&W in it, that's why I reload so I don't have make do. I guess I was more concerned with putting the tapper crimp on. I know that we are talking only a handful of thousandth with all the cases but I wasn't sure if that will change the grip on some. I plan on using this mostly for plinking fun with some deer hunting thrown in. Again thanks for the help. Peter