As mentioned, length, not weight is what matters for stability. Most 1/9 rifles will not stabilize the 75gn A-Max but will do fine with the 75gn BTHP which is a shorter bullet. The 77gn SMK is quite short for its weight and shoots fine. I have a 1:9 AR with a 16" barrel and a 1:9 Savage with a 26" barrel and both shoot 69gn Noslers and 75gn Hornady BTHP extremely well. They also both shoot light bullets very well. A shooter is a shooter, bullet weight and twist have precious little to do with that as long as the twist is enough to stabilize the bullet.
^^^and I would hardly call that button rifling job "standard." That was done with a worn cutter that chattered all the way down the bore.