I hear the 5.56/M198 is good for the first 100 rds to break in a new gun.
Curious on that. Seems just about anything that squirts a common projectile out the noisey end reliably would work for general rifle break-in. (NO- I'm not about to touch the barrel break-in thing again.
) I bought a R-Guns upper some months back, put it on a new Doublestar(?) lower. I ran regular 55gr brass PPU ammo through it. After 40 rounds- it had no sluggishness or short strokes or anything else I might have found objectionable.