the .68? is the bore of the barrel i.e. the inner diameter of it. nothing to do with what gun they fit. changing the bore of a barrel to match the size/brand of paintball used. this generally gives better efficiency and consistancy over the chono.
The determaning factor of which barrel fits your gun is the thread, these are inmost cases either autococker which fit eclipse, dye , bob long , vanguard, mini/axe etc. Impulse or smartparts which fit all smartparts guns exept the sft shocker. Spyder for alot of the older budget guns and 2 angel threads one for the G7 and older and one for the a1.
hope this helps.
oh and a a5 thread for tippmans etc.