Will USGI mags accept Black Hills (or any other manufacturer) 77gr ammo without any problems; are there any issues with cartridge length?

Ain't this a kick in the head .

I had been puzzeling over the GP website trying to decide what to order when I thought of thr 77gr issue.

Quoted: Yes, as long as they are loaded to Mag Length. The 77 gr bullets are known as "LTB" or Length Tolerent Bullets.

Could you go into a little more detail?

I currently own a RRA NM A2, 1/8 twist. Shooting surplus 55gr is fun and inexpenisve, but it's probably like putting regular gas in an Italian sports car.

I'd like to experiment with various loads and bullet wheights to try and determine what shoots best. With that in mind I want to order mixed boxes of 50-100 rds. Perhaps some 62gr, 68gr and 77gr to start out with. Maybe one or two more (no moly). What would you suggest? Hornaday's or Sierra's?

FWIW, my rifle likes the 69gr Sierras from Black Hills more than the 75gr Hornady from B.H.  I've shot the 77gr Sierras as well (not from B.H.) with good results.  I'd say try the Sierra 77's for sure, and the Hornady 75's for comparison.  

B.H. comes in 50-round boxes, so I don't know if you'll be able to get a mix in a 50-round box.  It's good stuff, though.  Save a few bucks and get the blue box reloads.  Absolutely no compromise at all in performance over the red box.