You should ask here: www.ar15.com/forums/forum.html?b=2&f=415
Questions seem to get answered pretty fast.
The Ares supplied bolt catch is like a standard one with the paddle cut off. Hard to hold the carrier back since it's so small. Your choices are Wes or Tom Cassidy (both post on subguns) have a nice one for $40 to $50. Or use CMMG's and cut off the paddle as Toad has suggested. I have Tom's.