You could put the accessories (optic, bipod, sling) on their own texture sheets if they're genuinely modular pieces that could be re-used on other weapons.
For an example of a modular character design, check out Jesse Sosa's excellent example http://oldwiki.polycount.com/SkankerzeroModularCharacterSystem (the images aren't up on the new wiki yet)
Thanks for the replies, everyone. Modular seems like the way forward - back to work for me. Time for a pot of coffee and some Boards of Canada, I think....