Cobrad,
The factory Glock barrels are machined with the chamber dimensioned towards the "loose" side to aid reliability. As with any mass-produced machinery, some are "looser" than others which really only matter if you want to reload your cases. The aftermarket barrels' big advantage usually lies in having conventional rifling instead of the polygonal used by Glock, thus allowing practice with reloads using cheaper lead bullets, as well as having a tighter chamber. This makes for the chance of having a more accurate pistol, but I have stuck with the OEM barrel in my Glocks because I feel that absolute reliability is better than shooting target groups 1" smaller. Just my 2 cents.
HP