I had the same problem when I got mine several years ago, and I was going to cut it down. Fortunately, I called Bushmaster and explained it in detail to the rep, and he had me send it back for exchange. There are two different lengths available, and you want the carbine version. If they still have the two barrel lengths, ask your rep to open the box and measure it.The barrel should be 11 7/8" long, and OAL should be 15 1/2 inches from the back of the receiver to the muzzle.
I didn't like the way it fit the rifle, so I had to make my own mounts. It fits ok now, but I have never fired it, and I keep it put away because it's kinda dumb, if you think about it. What are you going to do, show up at the range with it mounted? I guess I bought it because it seemed kinda cool at the time. It actually DOES look cool... now you got me thinking... I'll have to get it out for a few pictures.