I decided today to go back and get the three reaming achievements I need which are the challenge ones... but I get on to see that XBOX.com says I haven't unlocked "Master of the Arsenal" but when I get on my xbox it says I earned it and its unlocked but after tallying up everything I will still be missing 25g and that is how much the master arsenal achievement is worth. Is there anything I can do to fix this?

Do it again maybe. Or contact microsoft about it.

Yea my brother thought of that as well, since I started a new game to get the GRA challenges done I will try to exclusively use the normal GRA weapons