Protect finishing of our slide

I hope you can help me: I like to keep the matte black finish of my guns/ rifles, I need a clear coat and I have on sight 2 clear coat, both from the same brand (rust-oleum) one is the Ultra cover 2X, and the other is the Polyurethane clear coat, btw those 2 are the only clear coats that I can found near in the market. Should I buy one of them? Or they will not work?