As a collector who lives in Brazil and buys a lot of PB stuff in the US, I can tell you how I've been transporting it over the years (just got back on monday with two complete markers and several other parts, including a new Micromag 09 body kit and a VER frame for my Chord Mag).
As others said, you can only transport the marker in the checked-in luggage and you must remove the valve from the tank. And that's about it. You don't have to declare it, you don't have to disassemble it (in fact, it's better if you don't as a paintball marker overall shape is very different from a real firearm and if you do, all they will see on the X ray will be a gripframe which can actually be confused for something it's not).
I've made several international travels with LOTS of markers in my bags and NEVER had a problem. I've already transported 10, yes, I said ten markers on my bags and nothing hapened. I've been stoped when I was geting into the airplane and questioned if I had a firearm on my bag (happened with an eMag). I explained it was just a paintball marker and that was it. I've been questioned once in the Miami airport by three FBI agents and again I explained it was a paintball marker. They asked me to see it, I've shown them and we all shared a few chuckles. They were doing their jobs and I was cool as I knew I wasn't doing anything wrong. I've been stoped in the Brazilian customs and asked to open my bags with 6 or 7 markers inside. I pulled one the markers and explained what it was and the agents let me go with no further questioning or problems.
The important thing is to remain calm and confident, answer the questions promptly and firmly and efectivelly KNOW you are not doing anything wrong by just transporting your daily paint slinger LOL.
Have a nice trip.