minimum brass trim?

[Rhino0427]

Okay, so this is kind of a newbie question, but I'm beating my head against the wall. I'm trying to get a baseline for handloads for a new AR-10 I built with the help of a great gunsmith. I got some PMC brass that I shot through a 1919 MG as a birthday activity. When I was ready to make an initial round of cartridges for the new gun, I sized it (RCBS) and trimmed it (World's Finest Trimmer). As I trimmed it, I noticed a huge range on the trim. The "trim to" length is 2.005" and I was getting ranges from 1.990-2.010, which was really strange as I've had good luck with the WFTs before with other brass. After getting enough brass for an initial round of 50 at around 2.000" trimmed length, I trimmed 5 more pieces of brass (again with a wide range) and sent the trimmer back to the manufacturer to adjust, as the instructions say to do. After a few days, they called me and asked if I had cammed over when sizing the brass as they aren't used to seeing such wide variations from their trimmers. I told them I had and they said they would adjust and get it back to me. Meanwhile, I finished my gun and went to shoot it for the first time. Simply put, the bullets didn't work, they wouldn't load right or allow the bolt to close completely. I had measured every part of the cartridge and it was all well within tolerances, so you can imagine my frustration. As I was about to quit and go back to the smith, I went ahead and bought some factory ammo in the same configuration (168 gr. HPBT from Federal) and it worked like a charm. Okay, so my bullets are bad.

My trimmer just came back with a note that said it was good to go, they even tried some of their own brass to check it. After carefully resizing some of the PMC machine gun brass (not previously sized or loaded) and all 20 of the FC brass, I put them back in the WFT trimmer. The PMC brass was really short and showed a lot of variability again. The FC brass was much more consistent, but when I checked it, its all in the 1.986-7" range. So, two conclusions:

-The PMC brass is possibly beyond my range to size it as the problem seems to be the shoulders from using it in the 1919. Okay, no worries, I order more brass and punt on the machine gun brass
-The trimmer trimmed all my Federal brass (that worked before) really short, but consistent. Okay, so the shoulder is fine, but I probably need to adjust the trimmer so it trims in the 2.000" range.

So, all of this is a very, very, very long winded way of asking, "Is the FC brass too short now or is it okay to load and shoot recognizing that it is not at "optimal" trim length"? And no, I'm not really stressing over 20 pieces of brass. I'm just trying to learn as much as possible about a really nice platform that I built, but don't have much experience with.

And if any old timers have any input on the rest of my tale of woe, feel free to weigh in...

[bobbyfairbanks]

.308 doesn't have a whole lot of neck. But if you can still get good neck tension, shoot it. I just wouldn't expect this run of brass to give you great groups. Load it, save it and shoot it up at my next tac rifle course.

[Jamnanc]

it's not too short, but you do want it to be somewhat consistent for accuracy. Do the old, push the bullet tip into the bench, hard, and make sure it won't move.

[Rhino0427]

Will do and thanks for the info!

[rondog]

Maybe you could try annealing the brass first?

[Zombie Steve]

Trim length usually just isn't all that important. It matters, but I think you have a sizing problem. You should be able to push the shoulder back in place with the sizing die. I usually raise the ram, turn it down until it touches and lock it in place. I have, on occasion, had to cam over slightly. Case gauge is your friend.

I'm not going in to work today, and I have bourbon. Throw the chains on the truck, bring some sized brass and we'll take a look.

[Rhino0427]

Dammit, just saw this...maybe we can take a look tonight beforewhile we have a refreshing beverage...

Trim length usually just isn't all that important. It matters, but I think you have a sizing problem. You should be able to push the shoulder back in place with the sizing die. I usually raise the ram, turn it down until it touches and lock it in place. I have, on occasion, had to cam over slightly. Case gauge is your friend.

I'm not going in to work today, and I have bourbon. Throw the chains on the truck, bring some sized brass and we'll take a look.