Once you have your approved Form 1 back, I'd take it to a gunsmith to do it for probably $100. You could take a hacksaw to it, but then you're going to end up with one ghetto-ass looking gun.
But you're carving up a nice shotgun and you're going to pay a $200 tax on a gun that I assume you're going to saw the stock off too. But you're essentially making a Smooth-Bore Pistol AOW.
On the other hand, you could just go pick up a Mossberg pistol gripped 500-series AOW which will transfer to you with a $5 transfer tax instead, for about the same total price. Just something to think about.
"Our contract called for 16 cases of rifles and ammunition for $10,000 dollars, not a machine gun...........That is our present to the General"-Pike Bishop
“When fascism comes to America, it will be wrapped in the flag and carrying a cross.”