If you are looking for the select fire AA-12 and you are in the US, It ain't
gonna happen. The AA-12 is a post 1986 design, and the only way you are
going to get hands on one is as a Class 3 dealer sample.
If you want a magazine fed gas operated 12 gauge, look hard at the Saiga.
Several outfits are doing some neat stuff with the platform for Multi-Gun
competition. Here is one:
R and R Targets