I've done several to remove the cleaning rod muzzle funnel. Use a loop of masking tape to get a good square line all the way around . Cut it w/your hack saw about 2 blades long of your line ,incase the blade wanders or otherwise gets out of line. The cut will be much easier than you think its going to be. After you've made the cut grab your. Flat file and square the muzzle back up to your tape line. Roll the out side of the bbl to a desired shape. I like a 9/16 drill bit here to ,by hand, bevel the crown . It only takes a couple of turns . Now you can hit it w/some emry cloth to clean up the tool marks and use your brass screw to lap and polish.
I've done a 12ga,30-06',8x57,and an sks. All had groups tighten up w/negligable changes in poi.
The best part is the current bbl length will give you the chance to practice a couple of times before you make that final cut for keeps. Which I will also suggest you do if metal work isn't part of what you do often.