this is with a 556 chambering
I thought we were talking about the .350. Rics post is spot on.Wasn't sure based on your first post if it was 5.56 or 350 Legend. Reference Ric's post #2.....those were ALL "rifle length" gas systems.
Do you happen to have the approx. time frame that IMR 4198 changed and the characteristics of said change?In the original M16 and Colt SP1 22 grains of old IMR4198 with the 55-grain M193 FMJ approximates ball ammo producing normal full-auto cyclic rate and velocity for 3200 fps from 20-inch barrel. Slightly exceeds SAAMI MAP in a .223 Remington chamber, but is not "dangerous."