First why 1 in 7 at all in a carbine?
what I think is this, cross platform use, IE, entry carbine, HB 1 in 9, lots of shots real fast, big barrel, most reliable ammo, and most damaging in a short barrel at close range, M193 ball, and M855 is also very fragmentary at sub 100 meter ranges.
Lighter barrel with 1 in 7, possible use with single shot heavy ammo for "sniper tactical" use, so you have the twist to feed single, long bullet, heavy rounds also, as well as using M855 for entry manuevers.
Im thinking if I want a cross platform rifle, IE, Accuracy beyond 200 meters, go with the 1 in 7, strictly a kick down the door and blast rifle, the 1 in 9 HB.
And seeing that I have a 1 in 7 HBAR full size already for long range use, and a light barreled SP1 carbine with 1 in 9 twist, the decisions been made.
6721, HB entry gun with the flat top for optics, retire the SP, shes tired after being a good rifle, then a Bad rifle, and now turning back into good rifle again, and shes not equipped with a forward assist, poor old girl, practically an antique.
Thanks guys!