I honestly can't think of any other console games with the level of firearm customization that Ao2 has, which seems really odd because I'd think people would love customizing guns to that level.
There are a few PC games I can recall where you could build a gun out of loads of different parts, one being Operation 7 which had one of the most extensive customization systems I've seen in any game.
You could take an FN FAL, make it into a high mobility Paratrooper type rifle with a short barrel, folding stock, and a 40 round mag, or make it a Sniper variant with a fixed steady stock, accurized long barrel for long range, and a high magnification scope. You could mix and match upper receivers between AR rifles, like an M416 upper with M16 parts or vice versa. On top of that it had loads of optics, and not just stuff like "red dot sight" or 'acog sight", it had aimpoints, trijicons, eotechs, night sights, etc. and lots of different camos along with a system that let make your own camo. I'd love to see a commercial game with the level of customization Operation 7 had, but pretty much any game I've played with gun customization has fallen short in one or more areas.
I would definitely like to see games with deeper weapon customization, CoD4/MW2 had a bit of it, and Rainbow Six Vegas1/2 had a bit too but it was kind of bare being just a few attachments and predetermined camos.