Well P-990 I think that's partially true. I do have experience shooting an old Bushmaster V-match with a 1-9 24" barrel. It shoots 69gr. bullets without a problem but then it does have a 24" barrel.
What was partially true about what I said? A 1-7" twist .223 WILL stabilize up to an 80gr bullet, no matter the barrel length. A 1-8" twist .223 WILL ALSO stabilize up to an 80gr bullet, regardless of barrel length. A 1-9" twist .223 barrel is perfectly adequate up to a 70-75gr bullet, regardless of barrel length. Some report getting acceptable accuracy up to 77gr bullets with longer 1-9" twist barrels, but that's not a hard and fast rule. I expect my 20", 1-9"-twist CZ will shoot OK with the 75gr Hornady HPBTs I have loaded up. The CZ does shoot just fine with 69gr HPBTs, like your Bushmaster does, even with 4" less barrel. The long 80gr HPBT bullets WILL NOT stabilize in a 1-9" twist .223 barrel, no matter how long the barrel. It's really quite simple.
All bullets get blown around in the wind, and 20-30 mph winds will do a number even on the long, sleek 142gr 6.5mm match bullets from a hot rod like a 6.5-300 Weatherby. That's just physics. Better BCs and better velocities help mitigate errors in wind judgement, but the bullets will not fly "straight and true" in those kinds of wind conditions. Compensation will be required.
And with a 16" barrel, your new AR won't be getting high velocities out of the longer 75/77gr .223 bullets anyway. My 16" Colt AR-15 chronographs right at 2500-fps with a max load of Varget under the 75gr Hornady HPBT. I could only drive 77gr bullets to about 2650 with my 20" Service Rifle before pressure signs reared their ugly head. I liked the 69gr match bullets because I could get a bit more speed out of them, (around 2800 fps if memory serves) and they were cheaper too.
std7mag, I used to consider Varget and RL-15 interchangeable for purposes of what bullet weights and speeds I could run with them. I tended to pick whichever one I could put my hands on during The Banic. H4895 works well too, though I thought it left my cases and rifle notably dirtier.
