Bore
The bore is the interior diameter of the barrel. The bore properly matching the brand of paint being fired is the most critical aspect of a barrel, far beyond gas effiency or silence in most situations. A poor bore/paint match will result in velocity variations, which causes inaccuracy. Two and three-piece barrels allow the barrel bore to be matched to the paint diameter without entirely new barrels. Paint to barrel matching is doubly important in closed-bolt markers (as ball detents in such markers serve to prevent double-feeding, not to prevent roll-outs) such as Autocockers, because small paint in a large barrel can roll out the front of barrel.
Spin barrels
The Flatline barrel, manufactured by Tippmann is designed to decrease the parabolic travel of fired paint. The barrel is curved such that an accelerating paintball contacts the top of the barrel, imparting backspin. This backspin generates lift (known as the "Magnus Effect"), resulting in a flatter arc and longer range. This is especially beneficial in woodsball or scenario paintball where overhanging branches limit the range of traditional barrels. It is available for the Tippmann Model 98, 98 Custom, A-5, and X7. There are conversion kits to allow the use of the flatline barrels on some other markers, as well.

The Apex barrel, manufactured by Ben Tippmann Paintball Design, also imparts spin to the ball. Unlike the flatline barrel, the Apex can impart back, top, or sidespin (this allows balls to arc around some obstacles, or have them drop over bunkers). The magnitude of spin can also be varied, allowing for a gentle curve or a sudden hook. It is available with threads for most markers. There are many ways to fit an Apex tip onto a regular barrel, ranging from "Ghetto" ways such as electrical tape, to sleeves that go over the tip of the barrel, sold at Underground Mod Shop and Palmer's Pursuit Shop.
Criticisms
Both the Flatline and Apex barrel systems, while having a significantly increased long range, do not increase the force the paintball impacts with as range increases. The paintball is able to move farther at a lower speed, but the chance of the paintball breaking is diminished greatly. The overall effectiveness of these barrels is called into question then, as a player will be tempted to seriously utilize the extra range of the barrel at a distance where the paint will not break open, as opposed to using these barrels for psychological and area denial effects.