9mm COL question

[Thane_Of_Fife]

Hi everyone I'm pretty much brand new to reloading and, though I haven't had any challenges to my other calibers, (45ACP, 45 Colt, 30-06, 44 Mag, and 308), I noticed a potential problem with 9mm.  According to my Lyman 49 book, "This cartridge headspaces from the mouth and therefore case trimming must be uniform and accurate. Do not reduce cases below the trim-to length...".  Lyman goes on to illustrate a prefered Case OAL of .754 in. and a trim-to COL of .751 in.  A nearby gun shop sells once-fired brass which I gladly use and, thus far, haven't had any issues with their 45 ACP and Colt brass I've loaded.  My method of case prep in terms of case OAL is to set my caliper to the max allowable OAL -.002" and if a case passes through it fine then it's cleared for the next step, otherwise, I trim it.  When I saw this note in the Lyman, I proceeded to measure each of 203 cases I have and see if any of the batch are under .751 in.  Herein lies the rub; the mean COL of the batch is 0.7398" with a standard deviation of .000919".  The max OAL was 0.7492" and a min of .7360".  For those who are as sick as myself, I've provided the normal distritubtion of the aforementioned measurements.  The ultimate question here is...can I use this brass?  If so, do I still need to maintain the prescribed OAL of the completed cartridge or would I seat the bullet .010" deeper than normal for a case which is .010" shorter than normal.  I understand that if I seat the bullet too deep, I'll generate too much CUP and blow the gun up...which I'm not interested in doing.
I appreciate your time and help.
 
Happy New Year
 
JD

[blpenn66502]

Yep, fired 9mm brass is almost always under most manuals trim length.  If you measure some factory rounds you are likely to find that the case length is also under the .750 length.  Go ahead and prep it to include removing any military primer crimps and load it up using the recommended COL.

[wncchester]

Case lenght is a seperate issue from OAL and no attempt to 'adjust' for case length is helpful. 
 
Straight wall rimless (auto) handgun cartridges headspace off the case mouth so we want our cases to be as long as possible and still allow reliable chambering.  Few of us ever trim handgun cases, they are almost always short by some amount but so long as the cartridges stop while allowing reliable primer ignition, it won't matter.   (And don't over crimp auto loads, that can - and often will - taper the case mouths so they can slide too far into the chamber, effectively making the cases work as if they were even shorter.)
 
(*) Small, hot pistol cartridge load length - OAL - can be important, sorta.  At their best, handguns are puny compared to rifles but a fast burning powder in a small place (like in a 9mm or .40) IS moderatly sensitive to the combution space IF the load pressure is high but not so much if the load pressure is more moderate.   Since there is little or no real ballistic advantage to hot handgun loads, if we can stay a bit under book max we can seat where needed in complete safety. 
 
The most critical need for any auto ammo is that it feed reliably and chamber/lock up consistantly.  Best OAL changes with various bullets AND chambers so what our specific rig may need may not agree with any book OAL anyway.  In fact, in some 46+ years of reloading for about everything that goes BOOM, I've never even looked at a book OAL (a lot of data when I started didn't even suggest an OAL).  I've always seated anything where it's needed and then developed my charge at that length.  I KNOW no book can tell me with any accuracy what my charges my weapon will or will not take safely so I use the book listings as a guide but not as if they are engineering tables.  And I don't hammer my auto handguns with hot loads that mean nothing in effect anyway.
 
 
(*)  Note: That OAL/seating depth-pressure thing is reversed in rifles.   By seating deeper we can allow the bullets a running start before hitting the rifling so peak pressure with a given charge is actually reduced by seating deeper, at least until the bullets are seated MUCH deeper that is practical anyway.