This may or may not help but, I got a SurShot for an 1100. As I remember, the forearm didn't want to fit and I got to comparing it to the original and noticed that there were a couple tabs inside the SurShot forearm toward the front that were keeping it from sliding all the way on. I removed the tabs with a Dremel tool and things went together as they should. Take a good look at the inside of the original forearm and compare it to the Surshot and see what you see.
Mark