The best, easiest method is to have a plater DE-plate the nickel.
In this process, a plater just reverses the process and the nickel and copper undercoat are removed.
People have tried to remove plating by mechanical means like bead blasting, but this often doesn't work well.
While the plating APPEARS to be gone, in fact there's still traces of it left in the pores of the metal, and when the gun is refinished with bluing or parkerizing, the remaining nickel makes the gun look like it has white freckles.