Minimum Case Length .300WinMag

Buck88 · July 15, 2009, 04:33 PM

I got a little hasty with the case trimmer on a couple of shells and can't seem to find any listings for minimum case length. The shells were cut, chamfered, and deburred and measure 2.606 and 2.605. Trim to length is 2.610. Should I just trash 'em?

**Unclenick** · July 15, 2009, 06:15 PM

Nope. You're fine. The SAAMI case length in the manuals is the maximum. The standard specification for all rifle cases that I've looked up SAAMI drawings for is that maximum number minus twenty thousandths (-0.020") as the minimum. So you are good when your sized .300 WM case has a trim length of anywhere from 2.600" to 2.620". Most trim lengths target the dead middle of that range to allow for tolerance errors and trimmer user alignment errors. With an adjustable manual trimmer, I usually try to aim at more like -0.015" from the maximum number. That usually lets me get about three reloadings before a sized case is found to exceed the maximum and has to be trimmed again. So, for .300 WM, I would be using 2.605" on purpose.

I also know of no penalty for being even shorter than minimum except less grip on the bullet and poorer resistance to rough handling. Also worth noting: non-uniform trim lengths lead to poor crimp consistency if you are roll crimping into a cannelure? They also encourage bullet grip variance, which affects accuracy by varying start pressure.

By the way, it is the sized case trim dimension that matters, whether you are full length sizing or neck sizing only. That is because sizing usually extrudes a case out a little longer at the neck, so trimming before sizing won't give you the number you will be shooting with, and that's the number that matters.

Buck88 · July 15, 2009, 08:16 PM

Ok Thanks. The cases were FL sized, i left that part out. Is it normal for the trimmed cases to vary about a thousandth?

**Unclenick** · July 16, 2009, 12:27 AM

Yes. You not only get slight misalignment in most trimmers, but you also get slight misalignment of the cases in the jaws of the calipers that can give you a reading that far off, too. The only way I know to minimize that other than long experience with the feel of the calipers is to put one of the Davidson casehead centering adapters on the stationary jaw.

If you buy some new unfired brass and measure the case lengths, you will find that unless it is Lapua or Norma or Nosler Custom, it is more irregular than a mere 0.001".

July 15, 2009, 04:33 PM	#1
Buck88 Member Join Date: June 30, 2008 Posts: 46	Minimum Case Length .300WinMag I got a little hasty with the case trimmer on a couple of shells and can't seem to find any listings for minimum case length. The shells were cut, chamfered, and deburred and measure 2.606 and 2.605. Trim to length is 2.610. Should I just trash 'em?

July 15, 2009, 06:15 PM	#2
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,738	Nope. You're fine. The SAAMI case length in the manuals is the maximum. The standard specification for all rifle cases that I've looked up SAAMI drawings for is that maximum number minus twenty thousandths (-0.020") as the minimum. So you are good when your sized .300 WM case has a trim length of anywhere from 2.600" to 2.620". Most trim lengths target the dead middle of that range to allow for tolerance errors and trimmer user alignment errors. With an adjustable manual trimmer, I usually try to aim at more like -0.015" from the maximum number. That usually lets me get about three reloadings before a sized case is found to exceed the maximum and has to be trimmed again. So, for .300 WM, I would be using 2.605" on purpose. I also know of no penalty for being even shorter than minimum except less grip on the bullet and poorer resistance to rough handling. Also worth noting: non-uniform trim lengths lead to poor crimp consistency if you are roll crimping into a cannelure? They also encourage bullet grip variance, which affects accuracy by varying start pressure. By the way, it is the sized case trim dimension that matters, whether you are full length sizing or neck sizing only. That is because sizing usually extrudes a case out a little longer at the neck, so trimming before sizing won't give you the number you will be shooting with, and that's the number that matters. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

July 16, 2009, 12:27 AM	#4
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,738	Yes. You not only get slight misalignment in most trimmers, but you also get slight misalignment of the cases in the jaws of the calipers that can give you a reading that far off, too. The only way I know to minimize that other than long experience with the feel of the calipers is to put one of the Davidson casehead centering adapters on the stationary jaw. If you buy some new unfired brass and measure the case lengths, you will find that unless it is Lapua or Norma or Nosler Custom, it is more irregular than a mere 0.001". __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

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July 15, 2009, 08:16 PM	#3
Buck88 Member Join Date: June 30, 2008 Posts: 46	Ok Thanks. The cases were FL sized, i left that part out. Is it normal for the trimmed cases to vary about a thousandth?