10-02-2022, 02:28 PM
6.5 Grendel forum has a lengthy, and I mean Lengthy, thread that might have some insights for you, although in your case what I would say is it is a combination of 2 things: small base die sizing + 2) Lapua construction at the head-rim/case wall interface. After 1 or 2 firings and resizes the case wall has been stretched some "non-zero" amount, especially with the small base dies. This 1-2 firings/resizing appears to be enough for the brass to relieve its sizing stress by crumpling at that Lapua interface. Now I'm sot saying Lapus is a bad brass or anything, it is great but its design apparently gets into trouble in this type of scenario.
I would recommend 2 things: 1. get whatever regular sizing dies you want, but not small-base dies. For an AR it's too much stress on any cartridge. 2. Get a few boxes of whatever 6Arc is selling, currently 108's and 105's, for 80-100 pieces of brass "not-Lapua"... after firing the factory round you can run some side by side comparisons of the 2 brasses in your new die and see what you find.
I have taken a few new Hornady 6.5 grendel cases I have on-hand, and trimmed/resized them down to 6Arc, they have worked for the 1 firing I did but I don't have any multiple firing-histories on them yet. I don't use any small base dies.
I would recommend 2 things: 1. get whatever regular sizing dies you want, but not small-base dies. For an AR it's too much stress on any cartridge. 2. Get a few boxes of whatever 6Arc is selling, currently 108's and 105's, for 80-100 pieces of brass "not-Lapua"... after firing the factory round you can run some side by side comparisons of the 2 brasses in your new die and see what you find.
I have taken a few new Hornady 6.5 grendel cases I have on-hand, and trimmed/resized them down to 6Arc, they have worked for the 1 firing I did but I don't have any multiple firing-histories on them yet. I don't use any small base dies.