I have some 75 grain Hornady bullets, I've had them for several years. My 1/8 and 1/9 shoot em' fine. I just don't ever shoot much of anything over 62-64 grains.
If 1/7 is so great, why doesn't everyone use it. It certainly cost no more to make a 1/7 barrel than it does a 1/9 , 1/12 or any other rate of twist.
The 1/9 works great for 40 grain varmint bullets and everything else I shoot in .223. If I want to shoot anything that a 1/9 won't stabilize, I'll shoot another caliber. I don't get the fascination with 1/7 barrels.
The AR is a military driven platform. The military typically uses 1:7.
1:7 helps to maximize the potential of a small caliber anti-personnel round when the appropriate ammunition is used at certain distances through certain rifles.
Intended use of the AR is important and will dictate the appropriate twist. Civilian use may be much different than the military, but not all civilians shoot varmints with specialty varmint loads.
A 16 inch chrome lined, light/m4/gov't profile barreled AR is not a varmint gun.
Stepping up to another caliber in the AR means nonstandard parts and/or much increased expense.
I'll take 1:7 if given the choice. 1:9 isn't a deal breaker though.