FPS Games that feature weapon progression and customization in Single Player like the Call of Duty franchise

I've played Call of Duty on and off since Modern Warfare (the ancient version) and I'm starting to wonder if there are any SP games that sort of scratch the same itch of customizing your weapons and unlocking more parts.

A game that comes close is actually "Mothergunship" and I think it's tons of fun, so it doesn't even have to be -that- realistic of a game as long as the mechanics include tinkering and upgrading your guns.