Firstly - barrel cam or helmet cam?
The cameras are the same, attaching to your head will give more footage.
On your head you film what you look at - consider the side/position in relation to how you hold your gun when shooting, and don't just pick the other side as instead if half a hopper in shot, you get a tree/bunker.
Putting it on the barrel will avoid those obstructions, but will film a lot of ground or sky and be jerky when moved
On to the cameras:
You can go branded, non branded and Hong Kong rip offs.
Check out Techmoan on YouTube for reviews on numerous cameras
The key ones I will highlight are the obvious contour and gopro, but also sony and bulletcams.
Gopro got in first and have pretty much cornered the market for action sports and pov shots on tv.
But for paintball the contour is more suitable, gopros sit as a big box on anything you mount it on
The sony action cam beats both of them
The contour is paintball ready and survives being shot without a case
The sony and gopro both need to be in their waterproof cases. This affects sound but you can add 'non waterproof' doors to let in sound.
Bullet cams should be paintball safe, but if in doubt get one from plasmalights and you know that it is.
Extra features such as Bluetooth or wifi support, remote screens or linking to your phone and remote on/off switches let you check alignment, makes sure its recording etc but are not essential. What is essential is a cloth to wipe the lens. Even if not shot they will pick up splatter and moisture on the lens and can ruin a good shot if you don't keep cleaning
Memory cards and batteries:
Make sure you use a fast enough card or no matter how good the camera is the card won't keep up. Use class 10 cards, and ensure there is plenty of space for hours of footage - and make sure its empty before starting
No cameras battery will last a full day. Some can have the battery swapped (note that if this is the case you may be able to use mobile phone batteries for less than the cameras battery, but check the specs)
If the battery is not changeable get a booster battery pack and use that to recharge between games (even if it is changeable you can too up the current battery!)
There are good alternates to the big names, my first hd action cam was an aee camera with the same chipset as the gopro and for a fraction of the price it came with everything but a memory card - camera, case, lots of mounts, screen, remote control. Check out the Techmoan youtube channel for comparisons of cameras
Mounting - don't be a sheep and follow the marketing, GoPro has the image of the most mount options, but I have found the contour more versatile. You can use the side slide mount or it has a standard screw mount in the bottom so can be mounted to any commercial camera mount from clamps to tripods
