I have a Remington Mountain Rifle, Stainless LSS, in 260 Rem. caliber. Since my Lyman trimmer did not come with a .264 trim guide, I was "forced" to not trim the brass. As long as the brass was under max. length, the brass was reloaded.
Now, I did chamber the case mouth, square the primer pocket, and debur the primer flash hole using Lyman tools.
In short, in the light weight barrel rifle that SHOULD detect any slight difference, the 1st load put down a 3 shot group on paper under 0.3". Yes, that sounds like a load of farm animal by product, but from limited testing with two different bullets, this particular rifle in this particular chambering just does not seem to care that the case mouth is not square or the next case was slightly longer. All testing was done with flat base bullets, boat tail bullets might make a difference.
My guess is, unless the difference in accuracy is going to cost you $$$ or your reputation in competition, for those of us who just shoot for fun and/or hunt, trimming makes no real difference, just keep it under max. length.
Have Fun, Larry
For what it's worth... there have been several times when I needed to trim cases, and did not have the correct pilot. I found that the next size down generally worked OK if not spectactular. Better that then having a round go over pressure for being too long. Trimmed a few .338's with a .308 pilot, likewise 6.5's with .25 cal pilots, etc.
If the average group for the untrimmed cases is 1.71 and the average group for trimmed cases is 1.67, this is only a .04" difference. But in the conclusion it says there is a .9" improvement for trimmed cases. Is this an error, or am I missing something. Thanks
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