If the law in your state requires barrel length to be included on the registration form, I'm sure it would not be legal to shorten it.
Rather than shortening the barrel, it would be more likely that a gunsmith would just find a shorter barrel, and replace it. Probably at a cost less than cutting, and re-crowning.
Your best route mght be to just look for a gun with the barrel length you want.
Cheapshooter's rules of gun ownership #1: NEVER SELL OR TRADE ANYTHING!