For the USP I used a noise texture with a multiply node (math).
For the P99 it’s a very high defined scratch texture with the same node. I added a voronoi texture with a color ramp to give it more tons of colors (look more realistic).
When I will stop? I don’t really know Maybe three more or less, it depend if I get bored of this work later. Actually I’m working on a model which I failed two times! I’m trying differents methods, I hope that next week I will finally finish it. It’s a Smith & Wesson…
Here we go, I extended my handguns collection again with a H&K P30L. I recognized that it’s not photo-realistic, I was bored about realistic renders so I think it’s going to be my last pistol model.
Maybe three more or less, it depend if I get bored of this work later. Actually I’m working on a model which I failed two times! I’m trying differents methods, I hope that next week I will finally finish it. It’s a Smith & Wesson… 
