My 6.5 Grendel Load Test

[Kramerica]

All the needed info should be in the pictures. Load tested different powder charges. Did this test in 30*F weather at 100 yards. Would like one load to shoot in all temps from 0-80*F. It’s a deer and coyote hunting rifle, would like to have enough energy out to 500 yards. I thought there was very slight ejector marks on the 26.3 load (see pictures) but I don’t have a seasoned eye for that yet. Also thought bolt lifted ever so slightly heavier at the 26.3 load. Manual says 26.4 is max. Thoughts?
Edit: the first group for each load a magneto Chron was attached to the barrel. It definitely shifted the POI, I never adjusted scope throughout this test. 26.1 didn’t read a velocity so I just guessed what it probably would have been.

 

[Dusty Stevens]

26.3 you still aint close on pressure

 

[Laurie]

I found slightly hard bolt lift on my Howa Mini Oryx with 8208 and the 120gn Scenar in initial loads whilst those loaded with the same bullet in the same brass (Lapua) with Ranshot TAC had flattened primers but very sweet bolt lift and extraction.

After examining the fired cases and comparing them to new ones, it was apparently a case body 'fit' / fireforming issue. New Lapua brass was around 10 thou' 'short' on base to shoulder compared to those fired with 8208 using a Hornady case-comparator whilst those fired with TAC had fireformed by around half that, and apparently needed a bit more shoulder movement. I can only assume that there is a tendency amongst case manufacturers to be 'generous' in clearances given the overwhelming use of this cartridge in AR-15 type rifles to date.

My hypothesis on all this is that 8208 with its 100% case fill-ratios was really banging the shoulders forward hard to the point where they became a slight crush fit causing the harder bolt lift - there were circular witness marks on the case-head where the bolt face had turned against them on unlocking, same as you get on a full-pressure neck-sized case that needs a bit of shoulder bump.

So far as loads go, bear in mind that US SAAMI maximum is a mere 50,000 psi whereas European CIP allows another 8,000 psi +, so no maximum SAAMI based charges are going to cause over-high pressures in a bolt gun, even the downsized Howa 'Mini' and Cz527 'Mini-Mauser' models with decent brass. (I assume that the reason for this discrepancy is back to the AR issue - not that ARs don't have strong actions rather that the locking lugs have to be machined pretty far down to accommodate the significantly larger diameter case-head of the 7.62X39/Grendel. I read somewhere that military trials with M and CAR-16 7.62X39 adapted rifles saw lug cracking appear at ~9,000 rounds, sometimes less.)

I couldn't follow up with IMR-8208 XBR as it was using up a bit of old stock and we can't get this powder in Europe any more. We can get Alliant AR-Comp, although it's currently as rare as hen's teeth, so that may be a possible alternative to Viht N133 and Ramshot Tac once I've had the chance to try it with 120-130gn bullets.

 

[Kramerica]

26.3 you still aint close on pressure

Well 26.4 is max according to the manual, I don’t like to go above max, especially being somewhat new to reloading. Which load would you guess would still be safe at warmer temps? I’ll check again in warmer weather but I’d hate to start getting all my data with a certain load then have to change it in the summer and start over.

 

[Dusty Stevens]

I’d hate to start getting all my data with a certain load then have to change it in the summer and start over.

Youll have to anyway. Books tell you a good start, your gun and targets say when to stop