In keeping with the realism and scale of different gun models, airsoft guns feature at least 2 barrels to standardize the ammunition type airsoft guns all use - 6mm BBs. The two barrels are the external outer barrel that makes up the visible barrel component of an airsoft gun and an internal inner barrel from which resides inside the outer barrel and where the actual BB's are propelled through. One can upgrade the outer barrel to a different barrel length to either shorten or extend the length of your gun allowing you the possibility of turning your airsoft rifle into a CQB set-up or a DMR/sniper platform. However to improve the performance of your airsoft gun, the inner barrel is the key here. The smaller the "bore" size or opening of the inner barrel is to the BB, the better the FPS, accuracy, and range of each shot will be. Thus when upgrading your inner barrel, choose a barrel with a tighter bore that will allow you to reap those performance benefits.