I bought a 1100 12 ga magnum as did my boss, they would only shoot the 3in mag. I ususally shoot the high load 2 3/4 for pheasant but they would not cycle. Put on my barrell from the regular 1100 and it would shoot the 2 3/4 shells. Asked the guy behind the counter at guns unlimited a gunsmith he told me same thing, magnums would only shoot 3 in shells. Sold the magnum still have the other one.
Thats my exp. for whatever it is worth.