A barrel has only one bore size. A kit comes with a variety of bore sizes. While all paintballs are of ".68" caliber, they many times range from low.68's to medium .69's in sizes. With a barrel kit, you can test a paintball and compare it to the different bore sizes (like .683, .687, .689, .691, .693, etc.). The closer the bore size of your barrel is to the caliber of your paintball will result in better/ worse accuracy.
Also, this should be in the barrels and paint section next time. Hope that helped.
