Well, first off, what do you mean exactly by you need "ideas"? It would help others to know if you're looking for what material(s) to make it out of, what sort of found objects could work in substitution for the components, that kind of thing.
As for the overall paint scheme for the BR, I'd suggest going with either all black, or perhaps Gunmetal Grey for the body and then a dull/flat black for the accents (ie. scope, vent underneath scope area, forward grip, trigger grip and butt-end of the rifle).
Using a dull or flat black tends to give it a bit of a "faded" look (flat black isn't as vibrant as gloss black), and in my opinion would better match the in-game render. Just my two-cents worth on that... ;-)