man you got me on this one. i too am sort of looking for a inexpensive pistol cal ar build.
did you get one of those "surplus" 9mm h-bars form mil-surp.com?
being a pro mag i would run screaming away from that one. though keep in mind i am only going by the name/ pro-mags are known to suck.
C-products makes a mag thingy for $80 palmetto state armory sell them.
spikes has their mag block on sale for $130 last time i looked. it is pricey but it is adjustable and might be worth the extra $$.