Personally, I would buy a case gauge. They're handy. Yes, you can use your gun barrel, but I usually make a lot of rounds when I reload, and I can sit out on the patio with a box of ammo, my case gauge, and can plunk test them pretty quickly. And then I don't have to reassemble my gun. Maybe it's lazy, but the case gauge is handy for me. And it has been milled to a gnat's a$$...it's dead on for spec.
If you're putting an unscratched round in your gun and chambering the round, and then ejecting it and now it has a scratch, it sounds like you maybe have a rough bit on your feed ramp or the magazine.
U.L.