Conversion of 6mmARC brass back to 6.5 Grendel - Lazy's test results

[lazyengineer]

lazy,

To correct the head space, try expanding the neck up to 7mm, then back down to 6.5 in the FL die. This should form a false shoulder for the case to seat on. I have done this in the past with other calibers and it works very well to control head space and eliminate case head stretch.

Then of course one could also order a special hydraulic die to fully form the shoulder where it needs to be.

I thought about that. the appeal being easier and likely more concentric for the fireforming. My next step up though from 6.5mm is 7.62mm, and that's just a really big jump from 6mm. I was worried about trashing the brass. I also wasn't sure if that would be enough shoulder contact to prevent case-stretch during fire, to fill the headspace.

it might work though, it's not a bad idea. being fresh annealed, it might work, but for now i slightly prefer just swaging down the case body..

[vaguru]

If I was going to expand up, I'd do it in steps, not all at once. If you only have 7.62mm, do 6,5mm first, then the 7.62mm, should be less "stress".

[Zeneffect]

I'm surprised you got as far as you did without checking the resulting neck wall thickness after expanding. Either way, maybe a hydroform out to 6.5g would be better than fire form.

[Fess]

The 7.62X39 brass did in Grendel what it often does - it shots just way WAY better than factory 6.5Grendel brass. I see this again and again. I think it's due to the thicker neck you get running .30 cal brass swaged down to 6.5mm. 6.5Grendel chambers are sloppy in the necks, and I suspect 7.62x39 sourced brass converted to 6.5 Grendel, compensates for that. Don't know, but it's typical for me to see this performance improvement by using 7.62x39 brass. Recall again, this is the fireformed 7.62x39brass on it's 3rd or 4th run, but even during fireforming I'll get my best groups with my Grendel.
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Very early in the Grendel's life, Arne Brennan and his company (Competition Shooting Sports?) had a variation with a tighter neck that was initially called something like "Match Grendel". The name went over like a fart in a scuba tank on this site so it got renamed to something like "6.5 CSS Match" or maybe 6.5 PPCX. Although my memory is vague, I seem to recall that the neck was 5 thou tighter.

[VASCAR2]

Lothar Walther sold 6.5 CSS chambered barrels. My Son in Law has a 22” Lothar Walther AR-15 barrel with that 6.5 CSS chamber and it does have a tighter neck. Some of my reloads would not chamber in his barrel even though they fit my 6.5 Grendel AR-15’s and my Sheridan 6.5 Grendel case gauge. I think the 6.5 Grendel is .300” neck and the 6.5 CSS is .295” but that is just my recollection.