Rewards for prestiging a weapon are purely cosmetic. At 1st prestige you can add your clan tag to your weapon, at 2nd prestige you can add your emblem to it & upon completing the 2nd prestige you get a new calling card.
NOTE: camos do NOT reset when you prestige your weapon, you only lose its attachments.
Also when you prestige, your attachments are reset like rehab said but that allows you to get extra Xperia for earning the attachments again. So not all cosmetic, you can earn more xp which will help level you up.