I think that if you are going to practice painting, what you should do is go to home depot, buy one of those sheets of plastic or go to walmart and buy a clearsoft and practice your design on that. If you do get the clearsoft, you only waste like $20, but you could probably have a matching gun to go with your future AEG.
And yeah, you'd be better off just buying a cheap mechbox and then practicing on that. If it breaks, you won't be heartbroken. My friend tried to practice on his own M4 and broke it, so now he is stuck with a paperweight and no working gun.
Ares M110 LEAF - Ares Striker Tan
TM G17 Pink - KJW P226 TwoTone