A One piece straight bore barrel gives you improved accuracy at an affordable price. Includes infamous barrels such as the Lapco Big Shots and Smart Parts Progressives.
A multi piece barrel or barrel kit lets you change your barrel bore size without chaning your entire barrel. Tippmann Straightlines and Smart Parts Freak Systems
Flatline and Apex barrels give you more range. M16 and AK47 style shrouds get you the mil-sim look you want.