I have the AR 15/M 16 M261 conversion kit which I purchased from Sarco Inc. It may be a surplus military unit?
Anyway, it is a all steel unit and well made in my opinion. I bought it last fall for about $100 or so.
It uses a all steel 10 round magazine which fits into a standard AR 15 magazine. I forget how much the mags cost.
It is a very good unit in my opinion. Works reliably with all rimfire ammo. Installed it in my Rock River upper ( 1 in 8 twist) and with CCI standard velocity ammo ( which gave me the best results) it would put 10 shots into one hole at 50'. I don't know if the units are available now. For the money and the results I would seriously check it out.