Marauder Inc is an online retailer that sells many great 1:18 scale military weapons and accessories that are a perfect match for many 3 & 3/4" action figures, but certainly seem tailor-made for GIJoe figures. They have quite a range of weapons, some that are modular with detachable scopes or stocks or grips, etc (which is an amazing feat at that small scale).
Marauder Inc opened a Kickstarter project last year for a line of 1:18 military figures called Marauder Task Force that was a runaway success and hit many stretch goals beyond their original goal line. I pledged and I very much appreciated their dedication to providing frequent, informative updates and quality figures (now available on their storefront).
I'm a bit late catching up on this one, but recently in August they started a new Kickstarter project called Marauder Task Force: Valkyries which, for the most part, focuses on female soldiers. That should make many fans very happy, if their trend of quality continues. They've already met their initial goal line, but they still have 4 days left to unlock some nifty stretch goals.