Case trimming tolerances

[338LM]

What is the +/- tolerances used when trimming rifle cases?? ( 3030 win ) is the current case working on..

What is shoulder bumping?? How is it done??

I'm using the Hornady lock-n-load cam-lock trimmer... any helpful hints on using this equipment??

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[po18guy]

Since the .30-30 headspaces on the rim, case length is not critical. Overly long cases might prove difficult to chamber, but short cases should not pose a problem. "Shoulder bumping" is done to adjust headspace on rimless and rebated cases which headspace on the shoulder. The .30-30 essentially fireforms itself with each firing and the chamber determines where the shoulder will end up. What is the problem or concern?

 

[ma96782]

If it'll make you feel any better.


Taken from.

SAAMI Rifle Cartridge Catalog | The Ballistic Assistant

www.theballisticassistant.com

Aloha, Mark

 

[Mikej]

What is the +/- tolerances used when trimming rifle cases?? ( 3030 win ) is the current case working on..

What is shoulder bumping?? How is it done??

I'm using the Hornady lock-n-load cam-lock trimmer... any helpful hints on using this equipment??

When trimming cases there is normally a .010" range. Some books don't show the range. The Speer 14 book gives a "Trim To" number and a "Max Length" number. Max length is generally .010" over "Trim To".
Unless you're really getting into perfection any length between those two measurements will be okay. Roll crimped rounds need to be closer to same length though. Such as .357 mag, .30-30 in a tube fed lever action ,and others.

As far as shoulder bumping, measure what a new, factory round's shoulder measures, with a headspace comparator ..

And then bump the shoulders of your fired brass to match.

 

[338LM]

My concern is, while trimming my cases I when under the 2.029 trim to size... I was wondering if I scrapped those cases..

 

[Mikej]

My concern is, while trimming my cases I when under the 2.029 trim to size... I was wondering if I scrapped those cases..

If you're not roll crimping there shouldn't be any worries, unless your trimming so far below "trim to" length that it affects the neck tension that holds the bullet from sinking into the case enough to affect pressure.
How much below "trim to" did you trim?
My book shows 2.030"-2.039" for .30-30 win. If you're shooting that in a lever action a variance of more than .003" will affect your roll crimp IF you're using the seating die to crimp. If you get a Lee factory crimp die it allows you a little more tolerance. I chose to get the Lee crimp die for the very reason to not have to be so exact with case length.

 

[Mikej]

I'll ad that, if you're shooting in a lever gun, you can make a dummy round to drop in the chamber to make sure your case and chosen bullet go in as far as they need to for proper function.

 

[338LM]

I'm using a rcbs seating die... I'll look in to a crimping die... the shortest case is 2.022.. my book showes 2.029 trim to size... 2.039 max size..

 

[338LM]

Thanks everyone for your responses..

 

[Koda]

What is the +/- tolerances used when trimming rifle cases?? ( 3030 win ) is the current case working on..

What is shoulder bumping?? How is it done??

I'm using the Hornady lock-n-load cam-lock trimmer... any helpful hints on using this equipment??

Im currently just starting on a 30-30 handload , I trim mine to 2.030". Ive had the same question on tolerance, but while I not certain it will make a huge difference on the 30-30s accuracy I know consistency is the key with reloading any caliber. I try to hold +/-.002". My guess is that large variations in trim length would correlate to larger variations in neck tension, thus accuracy. Again, not certain how much this would affect accuracy in the 3030 lever guns but its still good to be consistent.
Shoulder bumping is using a FL die to size the location of the shoulder only, the idea is to minimize forming the case just enough to reliably chamber, thus increasing case life and accuracy. Take a fireformed case and measure its shoulder with a comparator gauge. FL size it normal and measure the difference, then back the die off in the press until your only moving the shoulder back about .003".

 

[Mikej]

Im currently just starting on a 30-30 handload , I trim mine to 2.030". Ive had the same question on tolerance, but while I not certain it will make a huge difference on the 30-30s accuracy I know consistency is the key with reloading any caliber. I try to hold +/-.002". My guess is that large variations in trim length would correlate to larger variations in neck tension, thus accuracy. Again, not certain how much this would affect accuracy in the 3030 lever guns but its still good to be consistent.
Shoulder bumping is using a FL die to size the location of the shoulder only, the idea is to minimize forming the case just enough to reliably chamber, thus increasing case life and accuracy. Take a fireformed case and measure its shoulder with a comparator gauge. FL size it normal and measure the difference, then back the die off in the press until your only moving the shoulder back about .003".

It's possible that the .30-30 in a lever gun will not need any shoulder bump. The lever action being similar to a bolt action, the case will expand and then spring back the slightest bit. Check a fired case in the chamber before sizing to see if the rim seats properly. I found in my Swede Mauser I didn't need any shoulder bump! Yay, less working of the brass! A gas gun, in my case an M1 Garand, I'm needing to bump about .002".

 

[338LM]

Thanks again for all your helpful information...

 

[Koda]

It's possible that the .30-30 in a lever gun will not need any shoulder bump. The lever action being similar to a bolt action, the case will expand and then spring back the slightest bit.

Do you mean neck size only? You want it to feed reliably, (more similar to a semi). I thought all neck sized rifle cases eventually will need the shoulder bumped back?

 

[Pepe-lepew]

30-30 you do not shoulder bump like a semi auto or bolt action. Lever actions do not have the mechanical advantage or recoil spring to close the bolt on a tight tolerance. Look carefully at SAAMI shoulder measurement and you will find 30-30 has a generous shoulder clearance for this reason. Follow the SAAMI minimum clearances and your rounds will chamber fine and brass life will be decent. You have to do some math on the drawings to determine the clearances.

 

[Mikej]

Do you mean neck size only? You want it to feed reliably, (more similar to a semi). I thought all neck sized rifle cases eventually will need the shoulder bumped back?

In my case. What I found with my Swede Mauser was that a fired case would chamber with the very slightest resistance just before the bolt closed completely. If I recall correctly there was .003" difference between factory PPU-S&B unfired brass and my fired brass. So when sizing I have the die screwed out that little bit. That's sure going to help case life. Trimming is still needed of course.

 

[Mikej]

I thought all neck sized rifle cases eventually will need the shoulder bumped back?

I would say it's possible? If the brass loses it's elasticity eventually? But considering the brass is snugged ever so softly up against the shoulder, how will it get to the point of needing a bump? Plus, the way the die is set when sizing, IF the shoulder needed bumping, it would get bumping.

 

[Pepe-lepew]

I would say it's possible? If the brass loses it's elasticity eventually? But considering the brass is snugged ever so softly up against the shoulder, how will it get to the point of needing a bump? Plus, the way the die is set when sizing, IF the shoulder needed bumping, it would get bumping.

There is a lot of things happening. High pressures stretches the chamber and flexes the bolt lugs. It also pushes the brass to the limits of the chamber. There is spring back of the chamber besides the brass. I suspect this is why neck sized cases eventually need full resize. Lower pressure loads probably are not as sensitive. However lower pressure loads can also cause case shoulder shortening that may or may not size back out. All these things are characteristics of the firearm, cartridge, particular brass and load and may be unique to the combination.