Do your homework. Find the specifications in diameter of gas ports from an 1187 barrel in 21 inches. How does that compare with your barrel now?
I have an 1187 Police with an 18 inch barrel. Shoots what you feed it. I have always had a minimum of 1 oz loads with 3 1/4 dram eq. For Turkeys, you'd be using a stouter load anyway.
It isn't brain surgery, but just research it out first.
With 1187 barrels costing a few hundred, I would consider cutting mine also.
john l.