I had the same problem when using UMC with the stock magazine but the problem didn't repeat when using the same cartridge with a Colt AR15 mag, strange? (It also left a strangely large amount of brass particles on the inner wall of the receiver).
I found the best performing ammunition to-date in all mags is a 45 grain JHP Winchester load (it's $8 for a box of 40 in Wal-Mart), never had any feed problems and great, consistent, accurate groups @ 100 yards often as tight as 1"!
I found the best performing ammunition to-date in all mags is a 45 grain JHP Winchester load (it's $8 for a box of 40 in Wal-Mart), never had any feed problems and great, consistent, accurate groups @ 100 yards often as tight as 1"!