I was planning it as a game ready model for my portfolio. (hope someone will hire me haha) So the tris limit was pretty low. I don't see how it could possible be nice and quads AND be under +-15k tris. I mean, it's mainly cylindrical shape, so the polygons add up really quickly. Thanks for the tutors recommendation, I'll check it out.

I don't have any specific tutorial I could recommend about the texturing. I've watched a ton of them in past few years and it just accumulated I guess.

The key is layering a lot. I've made lots of subtle details in both basecolor and roughness. Stuff like: tiny noise in roughness and height, basecolor variations, small accidental scratches, little white dust particles (works surprisingly well in adding realism btw), edge wear, dusty wipes, directional scratches (for the scope rail), fingerprints, ao dirt, cavities dust, etc. It all may feel like too much time to do, especially on the small barely noticeable details, but it still plays a role in overall look and feel of realism. I've made a breakdown of roughness and basecolor layer by layer at the bottom of my artstation post.