MSAR will be announcing a new STG-556 variant at SHOT that uses AR-15 magazines.  MSAR will also be producing AR-15 magazines made out of the same polymer as their STG mags.  They should be available soon after SHOT.  I have personally seen a prototype mag and they seem to be every bit as good as their STG mags, which run very well.  They also fit every mag well we have tried them in. They will be priced very competitively with other polymer mags.