We don't know why he painted the guns, for all we know, there may be a reason. All his previous service has been great, let's just wait for him to get back and tell us, yes? Everyone's been jumping to assumptions, it may sound dodgy but all you're doing is making someone, who is potentially innocent, look bad.
For all we know it's Customs fucking with people again, they've been doing it more recently- he had submitted the guns before, in stock condition, and he was told to repaint them (information thanks to Garrett, who bought one from him). He then painted the foregrip alone, but the Customs officers told him 'it still looks like a real gun'. It wasn't his decision at all, and it won't happen next time.