I'm not sure but it looks really close to a Choate extension +5 would extend it to 28 inches. I cut my +5 down to match my 24 inch barrel making it a 9+1. Nordic and Choate both make magazine extensions for it. Nordic makes a +8, 17.9 inches in length, Choate makes up to a +5 and it extends to the end of a 28 inch barrel. All of which can be had from MidwayUSA and I'm sure there are many more suppliers.