Case trimming question

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Call Turner
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Case trimming question

Post by Call Turner » Thu Mar 23, 2017 7:29 pm

So I have been loading for about 45 years and I have always gone by the book , or what ever manual that I'm working with at the time. Most all reloading books have a drawn out pic of the cartridge that your loading, showing all of the dimensions of the cartridge at every point. Take for instance I'm looking at a 308 cartridge with a case length of 2.015 and a finished cartridge length of 2.810 for factory specs. So why would you have a trim to length of 2.005? I understand that brass stretches, but why would you not return it to factory specs?
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Re: Case trimming question

Post by hatchdog » Fri Mar 24, 2017 6:29 am

Funny, I just had the same thoughts myself. Last week I trimmed just under 200 .308 cases, all Federal from their 168 gr match factory ammo. About 115 were fired 3-4 times, 36 fired 2-3 times and 30 fired once. I didn't measure them prior to trimming but I found that with most when trimmed to 2.005 the cutter just barely touched the case mouth. I doubt very many were 2.015 or longer. I just measured a Federal .308 168gr Sierra Matchking cartridge (a bit awkward with the bullet still in place) but I got readings from 2.005 to 2.007 so it looks like Federal factory brass is trimmed to 2.005 also. This doesn't answer you question but I thought I would share my findings.

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Re: Case trimming question

Post by eph411 » Fri Mar 24, 2017 9:38 am

The whole thing with min and max brass lengths is that you're trying to prevent brass from getting too long to where it could potentially jam into the lands and produce overpressure while at the same time keeping it long enough to provide adequate neck tension. The numbers in most published manuals then are .010" below the SAAMI max for that cartridge, but if you actually go reference the SAAMI cartridge and chamber drawings specs then they set minimum length as .020" below max. So all those numbers really reflect are the range of lengths that are deemed safe while still allowing for manufacturing variances.

Call Turner
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Re: Case trimming question

Post by Call Turner » Fri Mar 24, 2017 12:21 pm

But not all cartridges are called out to a different trim to length. The 300 Weatherby mag for example calls for 2.825 which is also the same as the factory measurement. But most being .010 less than factory. When shooting paper patched bullets you want your case the full size of the cartridge specs, or what ever the chamber measures or you will get a ring on the end of your case. Some folks in the match shooting world claim this can also ring your chamber.
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Re: Case trimming question

Post by eph411 » Sat Mar 25, 2017 9:28 pm

Call Turner wrote:But not all cartridges are called out to a different trim to length. The 300 Weatherby mag for example calls for 2.825 which is also the same as the factory measurement. But most being .010 less than factory. When shooting paper patched bullets you want your case the full size of the cartridge specs, or what ever the chamber measures or you will get a ring on the end of your case. Some folks in the match shooting world claim this can also ring your chamber.
Some of that differs from one manual to the next as well. Both my Hornady and Speer for example list the 300 Weatherby Mag at 2.825" max with a 2.815" trim, but the SAAMI spec shows it as being only 2.813" as a minimum length which is why other manuals will show no difference in trim and max length at all. This is one more reason I'm a big advocate of using multiple manuals to get any load data.

I think though your point is spot on in that how you handle trimming is really going to depend on what you're loading and what your desired results are for that particular load. I know for me personally I've tried chasing max accuracy by measuring my chamber length and trimming according to that chamber, but at least in my own rifles I've not found that to give me any noticeable accuracy improvement. So anymore I just trim my brass to uniform length and then go from there.

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Re: Case trimming question

Post by reloader762 » Sun Apr 02, 2017 5:20 am

If you want to know just how long the chambers neck is in your rifle to determine the best trim length for your reloads Sinclair makes a chamber length gage for $6.

http://www.sinclairintl.com/reloading-e ... 32925.aspx
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