Most fields and tournaments require you to use their paint. Standard paintballs are .68 bore, but the actual bore varies anywhere from .674 to .698. If your barrel is too small for the balls you're using, you'll break more paint. If your barrel is too large for the balls, you'll see a dip in consistency and in accuracy.
To get the best possible shot, a lot of paintball players use a barrel kit. Barrel kits contain inserts or backs that are designed to fit the exact bore of the balls you'll be playing with that fit into the barrel.
There are five different barrel threads that cover the majority of markers, and we carry barrel kits for all of them.
Autococker barrel threads were one of the first standard threads in the industry, and were used in the beloved Autocockers of old. Because the Autococker was the high end gun of its day, barrel manufacturers started using Autococker threads on their barrels, to the point where the became such a standard that most high end markers use Autococker threads even today.
With the introduction of the popular Impulse, Smart Parts started using a proprietary thread which they originally identified with that marker. However, once the ION proved even more popular, they began marketing their thread as ION. It's also sometimes called an NXT barrel.
When Tippmann entered the paintball arena, they did so with their own proprietary barrel threads, usually identified with their most popular marker using the thread. Tippmann's entry-level guns use a 98 thread, including their US Army guns. Tippmann's mid-range guns, including the A5 and X7, use their other bore, which was adopted for some JT guns, and was also used by most BT markers when Ben Tippmann spun BT out from Tippmann.
Lastly, Kingman's Spyder guns use proprietary thread of thier own, which due to their guns' popularity, have become adopted even more widely, including in the balance of JT markers.
If you need to more specific guidance on which barrel thread fits your gun, you can check our chart of paintball barrel threads.