I've sent a lot to the states, never had a problem.
For warranty work, make sure you list this on the outside and keep a copy of all the info,(even take a photo etc) so that when it comes back to you you won't have to pay taxes if you can prove it was the same thing that you sent out.
I've never had a problem listing it as "paintball marker" or "paintball equiptment"
The only problem I had was one was delayed for a long time as they decided anything over 2kg had to travel by boat, without telling me! month and a half later it arrived!