Nice build, those older Bushmaster barrels were pretty good shooters. Put an optic on it for a while and see what it can really do and for load development then yank it and go back to irons once you find it's favorite combination of projectile and powder.
Years ago I had a Bushmaster varmint with 20 inch 1:8 twist with wylde chamber and it was capable of sub moa with good ammo.
I have a build with an NEA upper/lower and they aren't even a matched set and the fit is very nice. Has served me well for a while now.
I just picked up an A2 stock and buffer tube with spring and buffer for $40 new, am thinking of doing another rifle length build.
This is so f'n addictive, I can't stop building AR's. Lol
50gr with 1:9 ??? I think you should go with 55gr or 62gr bullets with that twist...
Maybe beyond 300 yards but at shorter ranges any barrel twist rate can surprise you with what it shoots well.
Sweet build, I'm adding the final touches to my NEA build, how do you like the upper and lower? Any issues.
I was hoping to get a barrel with a 1/9 twist, but when my local gun store ordered it from NEA, I didn't know
the only made 1/7 twist. I have no idea how its going to shoot now.
Don't worry about the 7 twist, you can't overstabilize a bullet, all you have to worry about is that the projectile can handle the rpm it's going to have coming off the muzzle. Some highly frangible varmint bullets can't handle the rpm and can come apart as they clear the muzzle but those varmint grenade projectiles love to be pushed fast

My favorite is a 1:8 twist but any twist rate can shoot well. My 12 twist varmint rifle loves the 36gr varmint grenade and will put them all into a half inch at 100 yards.