Is it an AR-15 or AR-10? If so, you might effect the function of the rifle's gas system by cutting the bbl down.
Even if it isn't, you should have it done by a professional smith. Accuracy will go to shit if it doesn't have a proper crown. Crown should be cut concentric and square to the bore.
If the bbl isn't all that high quality to begin with, cutting it back might put your crown at a loose point in the bore. If so, accuracy will go to shit. Also, the bore might be somewhat off axis at the new point of the crown. When they drill the bore, it tends to curve slightly in one direction. When they first fit, chambered, and crowned that barrel, it was indicated in so that it is as straight a path as possible from either end. With a shitty barrel, cutting it back could end up making that barrel curve point in a direction that chews up windage or elevation.
There are a bunch of hand crowning tools that use pilots with bushings to achieve concentricity, but of course they're expensive.
Mainly what I'm saying is that just because you might find someone with a lathe, doesn't mean that they will have the tools or the know how, to properly cut back a rifle barrel and maintain accuracy.
Depending on where you live, you could check with many of the smiths in your area. I live up north, so Benchmark Barrels, is a place I would consider. Or perhaps Rick Freudenburg at Sam's.
http://www.freudscustomrifles.com/
"Cut and crown barrel.....................................$58.00 to $95.00"