To answer your question, 1:9 twist I would recommend 55-69gr ammo.
i concur, i own a couple 1-7 & 1-9 from different makers, i took 6 AR style carbines to the range and fired the exact same loaded rounds in them at 100 yds. there was so much variation in bullet impact that most people would not believe the difference, and believe me, there is a huge difference.
i used a Colt, 1-7, 1-9, Bushmaster, 1-9, 1-7 and LWRCI, 1-7, LWRC, 1-9 each one of those guns printed good results, BUT !! different.
oooooh !! i forgot to mention, all rounds were my reloads using LC brass CCI 400 primer and Ramshot TAC, i also repeated the test using Win.748.
someday i might try the test once more with BLC-2, CFE-223, & Ramshot X-TERMINATOR.
i enjoy trying different loads in different guns, i have found the nearly perfect load for 5 different AR-15 type carbines