again, ty did a good job summing it all up... personally, i wouldn't bother rebarreling it, unless the stock one is damaged or you want something longer. i run stock sheridan bore brass on all my primaries, following the 'if it ain't broke, don't fix it' saying... vert adapter is simple, a piece of valve tube milled to fit and soldered in place of the ammo tube, but then it'd be open class.
for bolts, i agree with stolz, speed bolts, either the original cooper-t or the sheridan knock-off, are better than sprung bb's. i use cooper-t underspin bolts in my primaries (again, as ty said, doesn't always work, depends on paint and marker). here's a quick pic for bolt identification:
1st: stock sprung bb long p-series bolt
2nd: long p-series 'speed demon' bolt
3rd: as above, but 'turned down' on lathe for smallbore brass
4th: k-series cooper-t 'underspin' bolt
5th: classic 'if it ain't red, it ain't right' cooper-t k-series bolt
6th: homebrew speed demon bolt for tightbore k-series
really, the first thing i'd do to make it more playable is add a fasstchanger...