Does your 18.5" barrel have rifle gas?
Pressure curves can be quite different with different powders & burn rates, so obviously the easiest thing might be to try another powder that is a bit slower..............you don't say how RL-15 or Varget worked, but that would be 2 powders that's I suggest you try with the 77's.
Also, if you have a rifle gas gun, I suggest that you use a carbine (3.0 oz) buffer for starters until you see that the gun will function reliably then go heavier if you are able to.
Lots of possibilities, lots of interactions; and a few 3 round tests don't really prove much.
MM
