Anything non-stock, but even my stock barrels work well with them
This is by far the best paint you can use for outdoor winter play. Just yesterday we went out (I live in WI) and I used Polar Ice, and I had no breaks or problems whatsoever. They are heavier than most balls, so they will actually fly straighter to some degree.
Not for use during summer, they are a hard shell and will sting if you get hit. You also increase the chance of breaks during warm weather. Use these balls only if the temp is under 40-50 degrees. I have seen breaks in the tippmann flatlines which can be annoying.
Overall, you should not have any problems with this paint. RPS guarantees every paintball that comes out of their factories, so if you do see some dimples or scars, let them know, they will replace it for you. The people who have problems with these might have left them in a car overnight or didn't take care of them, which would explain the dimples/seams showing. Have your local store check all your bags of paint before you buy them. Look for breaks in the bag or other inconsistencies before you buy. Easiest way to not get ripped off. Some stores have a bad practice of loading too many cases on top of each other or just throwing them onto the shelf. Just make sure your paint looks OK before buying.