Question About Case Mouth Squareness...

Naga · July 30, 2011, 01:48 AM

Hello everyone.

I just picked up an RCBS Trim Pro. It has been modified to run off of a power drill. I will be using it in place of my Lee trimmers, since it became a bear to prep a few hundred cases by hand after every range trip.

I noticed that RCBS claims that the Trim Pro produces very square case mouths. I decided to test this with my caliper, and I was astonished to discover a nearly +/-".004 variance around the mouth of each of the test cases! The difference was discernible to the naked eye when held against a measuring square.

Either way, I tracked this problem down to poor alignment between the cutter shaft and the axis of the casings when they have been placed in the holder. Through the judicious use of bits of spring steel as shims, I attempted to correct the issue. I have gotten it to the point that the variance is +/- ".0005 (the smallest increment my caliper displays.)

My question is this: How square does a case mouth need to be for good accuracy? Would the above stated tolerance be acceptable?

Thanks for any help

T. O'Heir · July 30, 2011, 02:29 AM

Hi. .0005" is 5/10,000". Not enough to make any difference. Neither is 4 thou. Square is good. Has to do with how the bullet leaves the case with a whole bunch of hot gases behind it. It's not something you'd measure or worry about though. Case length is more important. Trimmer shell holders aren't precision made things either.
You'll be chamfering and deburring the case mouth anyway.

Mike / Tx · July 30, 2011, 01:40 PM

For most of my rifle loads I use the RCBS Trim Pro with the three way cutter. I set it up to the minimum case dimensions and go to town. When done I inspect he case necks for anything out of the ordinary and get to loading or stash them away for future loads.

With my revolver rounds I check them every load simply to make sure they haven't gotten longer or out of square. If anything is noted it is usually the out of square part, and I will trim the whole batch just enough to square them up. I like a nice consistent crimp when I seat the bullets. This can only be your best when things are all the same length.

As for differing lengths, I don't sweat the OAL as long as it is at or under what it should be. Anything out by .003"will get another turn through the trimmer, this time it will be slowly turned in quarter turns to get it straight I hope.

TXGunNut · July 30, 2011, 02:55 PM

I'll agree that .0005" is better than .004" but not sure I'll ever be able to measure the difference on a target.

**Unclenick** · July 30, 2011, 03:04 PM

Unless you are sizing on a Lee Collet Die or in some other way are keeping the necks perfectly coaxial with the rest of the case, it would be hard to get a perfectly square trim. The manufacturers of new ammunition certainly don't.

July 30, 2011, 01:48 AM	#1
Naga Junior Member Join Date: July 30, 2011 Posts: 1	Question About Case Mouth Squareness... Hello everyone. I just picked up an RCBS Trim Pro. It has been modified to run off of a power drill. I will be using it in place of my Lee trimmers, since it became a bear to prep a few hundred cases by hand after every range trip. I noticed that RCBS claims that the Trim Pro produces very square case mouths. I decided to test this with my caliper, and I was astonished to discover a nearly +/-".004 variance around the mouth of each of the test cases! The difference was discernible to the naked eye when held against a measuring square. Either way, I tracked this problem down to poor alignment between the cutter shaft and the axis of the casings when they have been placed in the holder. Through the judicious use of bits of spring steel as shims, I attempted to correct the issue. I have gotten it to the point that the variance is +/- ".0005 (the smallest increment my caliper displays.) My question is this: How square does a case mouth need to be for good accuracy? Would the above stated tolerance be acceptable? Thanks for any help

July 30, 2011, 03:04 PM	#5
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,733	Unless you are sizing on a Lee Collet Die or in some other way are keeping the necks perfectly coaxial with the rest of the case, it would be hard to get a perfectly square trim. The manufacturers of new ammunition certainly don't. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

July 30, 2011, 02:29 AM	#2
T. O'Heir Senior Member Join Date: February 13, 2002 Location: Canada Posts: 12,453	Hi. .0005" is 5/10,000". Not enough to make any difference. Neither is 4 thou. Square is good. Has to do with how the bullet leaves the case with a whole bunch of hot gases behind it. It's not something you'd measure or worry about though. Case length is more important. Trimmer shell holders aren't precision made things either. You'll be chamfering and deburring the case mouth anyway.

July 30, 2011, 01:40 PM	#3
Mike / Tx Senior Member Join Date: April 9, 2000 Posts: 2,133	For most of my rifle loads I use the RCBS Trim Pro with the three way cutter. I set it up to the minimum case dimensions and go to town. When done I inspect he case necks for anything out of the ordinary and get to loading or stash them away for future loads. With my revolver rounds I check them every load simply to make sure they haven't gotten longer or out of square. If anything is noted it is usually the out of square part, and I will trim the whole batch just enough to square them up. I like a nice consistent crimp when I seat the bullets. This can only be your best when things are all the same length. As for differing lengths, I don't sweat the OAL as long as it is at or under what it should be. Anything out by .003"will get another turn through the trimmer, this time it will be slowly turned in quarter turns to get it straight I hope.

July 30, 2011, 02:55 PM	#4
TXGunNut Senior Member Join Date: January 16, 2010 Location: If you have to ask... Posts: 2,860	I'll agree that .0005" is better than .004" but not sure I'll ever be able to measure the difference on a target.