the bolts on tippmanns enter into the rear of the barrel. on properly made barrels, the rear is overbored to about .710 for about .5" into the barrel, to allow for the bolt to enter. this is why freak backs for tippmanns are longer than other backs, and why the inserts are not flush with the end of the back.
bolts jamming in small inserts / backs is not a tippmann problem, but rather out-of-spec barrels / barrel backs from the various aftermarket manufacturers.
Originally Posted by alpha434
Barrels are like girls... Some people are just happy to have one that looks good. Or even one that everyone else likes. Other people want a tight, accurate bore to run their balls through.