Though it predictively falls at the slower end of powders appropriate for the 6.5 Grendel, the Alliant AR-Comp powder has the attractive property of being temperature stabilized from -20 to +160 F. It has produced some phenomenal performance in my .300 OSSM, so I thought I'd give it a brief trip to the batter's box. It was assumed to have burn characteristics typical of Varget, but my experience in the .300 puts it between 8208XBR and RL-15. Definitely faster than RL15. Being an extruded powder, the specter of load density looms over it and in practice, it was indeed a limiting factor, much like my experiments with RL15 and Varget in this caliber.
Search as I may, I have yet to find data that can be plugged into QL, but have had some success starting with RL15 data and increasing the Ba. I started intentionally low in charge none the less and chose NBT 120s because I had some left over from my HBN testing, to use up. From previous tests, that coating costs roughly 5% velocity loss, so figure that into the results. I will probably retest with 123 Amax's just to see if it I can compete with the excellent results that 8208 provide with that bullet. 8202 seemed to be a little fast for the 123 Amax, so perhaps a happy accord can be found with AR-Comp. Fortunately, I didn't have to order the AR-Comp. A philanthropic source provided me with an 8lb jug of it to experiment with across different calibers. I never turn down a fee meal.
While I did not set out to find any absolute accuracy nodes in order to cover a wide range of charge weights for the limited number of NBT 120s I had on hand, I was nonetheless pleased with the results as they show promise. Range conditions were near ideal as the weather finally broke. I started out at 75 degrees and ended at 79, with low humidity and only a light head-on breeze. The trade-off for having a range 8 minutes from the house is that it is limited to 100 yards on the rifle range.
The issue of granule size and shape did not lend well to hign density loading, even after vibrating each charge on the variable vibration table to settle them in. You could hear the heal of the NBT moving grains out of the way, but not actually cracking them. I am no fan of compressed loads to the extent that I have a second-hand stethoscope and actually listen closely on experimental loads, for any signs of that. Yeah, I'm weird, but experience has taught me that predictive performance moves out of the linear phase and into non-linear when a load gets compressed. I don't like surprises.
So, here's a dalliance into AR-Comp
The 25.5 gr loads were used to sight in a scope change (Nikon Monarch 6-24x50SF) and were all over the place as I dialed in, so they are not included in the following accuracy images.
There was a little displaceable granule room left in the cases for the 120 NBT and with the right profile bullet, you might be able to get a little more in before it compresses. Going with non-HBN plated bullets will gain you back a little velocity as well. Given that none of the tested loads exhibited any pressure signs or short-strokes, there may be promise for this powder. The real test will be with the 123 Amax's.