Basically a fancy paintball gun/marker. The major difference being the magazine design and projectiles used. At the bottom of the page linked above, you can see what the projectiles look like.
If for some reason you really wanted to replicate the FN product filling these
yourself with whatever and just using a regular paintball gun, would probably be a lot easier and cheaper(assuming you could somehow get a 303 and ammo). Probably not quite as accurate or consistent, but workable.