Ha funny. I just got my grendel build done yesterday, 24 inch AR-stoner bbl. And also like you I bought a bunch of the honady amax 123. Good ammo, groups tonight were all 5 shots. 100 yards .737. 200 yards .906 and 300 was 1.747.
I am impressed with this grendel stuff. I have been doing alot of load searching, seems like AA2520 and win 748 are 2 of the top performers with the 120 -123 grain pills. I bought a can of 748 and a brick of mag primers. Havent loaded any yet to try though as i got 320 rounds of the hornaday ammo. I do have 200 new brass tho. cabelas has it on sale for $31.99 i think it was.
If you call Hornaday they will give you the equvilant load for thir ammo. They use differnt powder then what we can buy, but they will tell you what powder to sub to get the same velocity.