Quoted:
I am debating on whether i actually want an AR. I know they're like legos for adults but when I have uncle sam paying for all the .223 I shoot, I ask myself "why?"
I digress, assuming I buy one, it would most likely be a Smith, the M&P15T with 1:9 barrel. I have a few pmags of 75gr TAP loaded up. If there is a problem with the 1:9 barrel stabilizing 75gr bullets, at what distance do bullets destabilize. The heavy bullets would only be used in home defense at distances <50ft.
does anyone have real world experience with bullets destabilizing?

At 50 feet, I don't think it really matters much.

Quoted:
Why YES I do.  I worked on this with several 16" carbines at 25' preparing for an Appleseed shoot.  All the barrels are 1:9.  Staring with several 55 gr quality rounds, I had no happy.  The 5 round groups would not score worth crap on the AQT reduced (prone) targets - about 1" shoulder-up silhouettes.  The worst were the ... xm193, what a shock.

I worked up to 68 and 69gr Black Hills Match and they ROCKED.  One jagged hole, 5 rounds, repeatedly.

Since the 75s had never done much for me at 1:9 for 100 yards or beyond, I never went there.

Having said that, every rifle is different and has different preferences.  Testing is the only way to know.