75 gr hpbt will work just fine in AR mags. The military uses 77 gr SMKs in the mk262 load which was designed for an M4. I've shot close to 1000 rounds using 75 gr bullets through my AR without a single problem.
Veteran OEF (2002) and OIF1 (2003) - US Army
Member of the Burnet Gun Fighters, Inc
. and of course the NRA
Oregon State University alum -- Go Beavs!