When it comes to jacket walls, there are at least two types on the market: parallel and sloping walls. These come in various thicknesses, too. .012,.018, ,035, .065" and so forth, depending on the caliber. If you try to thin the jacket mouth yourself you run the risk of upsetting the center of balance, and thereby its accuracy. It's much easier to get some sloping walled jackets instead.
As to using an arrow cutoff saw, the blade is made to cut aluminum or carbon fiber, not copper. It may work for awhile, but too soon the blade will need to be replaced.