What Is SAAMI Case Trim Length 300 Blackout

brycealbright · December 30, 2014, 03:48 PM

According to SAAMI: 1.368-.020. This would be 1.348. My Hornady reloading book reflects: Case max: 1.368, case trim length; 1.358. I'm wanting to trim my cases to SAAMI length, what is it, just to be sure. Thanks.

TMD · December 30, 2014, 04:06 PM

Most people who reload this caliber including myself trim to 1.350

rg1 · December 30, 2014, 04:40 PM

I trim mine to 1.363" to 1.365". I just prefer to leave them longer. They don't grow much in length with what I've loaded. I shoot Hornady 110's and surplus 147 M80 FMJ's and crimp the 147's and my trim length is the perfect length for crimping them. Maximum length is 1.368".

**Unclenick** · December 30, 2014, 05:12 PM

Bryce,

You are misreading the SAAMI spec. The -0.020 is not a value you are supposed to apply to the first number; it is a tolerance. Cartridges and chambers have what are called unilateral tolerances instead of the more familiar ± tolerances. This is standard practice in engineering drawings when one end of the tolerance range is more critical than the other. In unilateral tolerance practice, the critical dimension is the number given and the tolerance is given in the less critical direction only. A cartridge that is too large can't fit or function at all, whereas one that is too small one still may, so the large end of the linear tolerance span is the critical one. That means all linear cartridge dimensions are given as their maximum value and the tolerances are minus only. For chambers it is just the other way around. For chambers too small is the critical dimension that could completely prevent feed and function, so the chamber's given linear dimensions are the minimum values with a + tolerance only.

For the 300 AAC Blackout, what the 1.368" -0.020" tolerance is telling you is that the case may be as short as 1.348" or as long as 1.368" and still be in spec. Most load manuals set trim length to the middle of the tolerance range, which would be 1.358" in this instance. This is on the assumption your trimming will include some amount of error and that number gives you maximum error room in either direction. Some folks, knowing they can hold tighter trimming tolerances than ±0.010", prefer to trim nearer the short end of the case length range so they can go through more load cycles before case growth exceeds 1.368", and thus they don't have to trim as many times in the life of the case.

skizzums · December 30, 2014, 07:46 PM

just to make life easy on myself, i trim to 1.350. it'll usually stretch a hair when you form, if you are cutting down .223. i am happy with them anywhere from 1.350-1.368 when finished.

but i understand if you don't wanna go that short when you start out

December 30, 2014, 03:48 PM	#1
brycealbright Member Join Date: October 12, 2014 Posts: 28	What Is SAAMI Case Trim Length 300 Blackout According to SAAMI: 1.368-.020. This would be 1.348. My Hornady reloading book reflects: Case max: 1.368, case trim length; 1.358. I'm wanting to trim my cases to SAAMI length, what is it, just to be sure. Thanks. __________________ "A 45 Makes All Men Equal"

December 30, 2014, 05:12 PM	#4
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	Bryce, You are misreading the SAAMI spec. The -0.020 is not a value you are supposed to apply to the first number; it is a tolerance. Cartridges and chambers have what are called unilateral tolerances instead of the more familiar ± tolerances. This is standard practice in engineering drawings when one end of the tolerance range is more critical than the other. In unilateral tolerance practice, the critical dimension is the number given and the tolerance is given in the less critical direction only. A cartridge that is too large can't fit or function at all, whereas one that is too small one still may, so the large end of the linear tolerance span is the critical one. That means all linear cartridge dimensions are given as their maximum value and the tolerances are minus only. For chambers it is just the other way around. For chambers too small is the critical dimension that could completely prevent feed and function, so the chamber's given linear dimensions are the minimum values with a + tolerance only. For the 300 AAC Blackout, what the 1.368" -0.020" tolerance is telling you is that the case may be as short as 1.348" or as long as 1.368" and still be in spec. Most load manuals set trim length to the middle of the tolerance range, which would be 1.358" in this instance. This is on the assumption your trimming will include some amount of error and that number gives you maximum error room in either direction. Some folks, knowing they can hold tighter trimming tolerances than ±0.010", prefer to trim nearer the short end of the case length range so they can go through more load cycles before case growth exceeds 1.368", and thus they don't have to trim as many times in the life of the case. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

December 30, 2014, 07:46 PM	#5
skizzums Senior Member Join Date: July 1, 2013 Location: Douglasville, Ga Posts: 4,615	just to make life easy on myself, i trim to 1.350. it'll usually stretch a hair when you form, if you are cutting down .223. i am happy with them anywhere from 1.350-1.368 when finished. but i understand if you don't wanna go that short when you start out __________________ My head is bloody, but unbowed

December 30, 2014, 04:06 PM	#2
TMD Senior Member Join Date: August 9, 2011 Posts: 1,293	Most people who reload this caliber including myself trim to 1.350

December 30, 2014, 04:40 PM	#3
rg1 Senior Member Join Date: November 6, 2001 Posts: 1,125	I trim mine to 1.363" to 1.365". I just prefer to leave them longer. They don't grow much in length with what I've loaded. I shoot Hornady 110's and surplus 147 M80 FMJ's and crimp the 147's and my trim length is the perfect length for crimping them. Maximum length is 1.368".