Dfariswheel gives good advice.
Some people use a chop saw, others use a hacksaw, and still others, like me, use a pipe cutter. I use a pipe cutter all the way through, and then polish things up with a piece of emery cloth. Once smooth, I finally burnish the muzzle with a large piece of steel, such as a ball peen hammer head. (Burnish, not hit!
) The burnishing is probably not really necessary, but making sure there is not an inside ridge is. Both ways work, the pipecutter just works better for me. Go slowly, and don't try to take to big a bite if you try a pipecutter. That will minimize the inner lip.
It is imperative that you do not cut the barrel to short. Take a wooden dowel, and insert it in the barrel with the SG in battery, empty, bolt forward. Mark the dowel at the end of the barrel. Remove the dowel. Measure the marked distance. This will be the true barrel length. Suppose it's 26 inches...... then you subtract 7.5 inches from the muzzle to achieve a 18.5 inch barrel. Use the dowel to assist measuring. DO NOT GUESS!
I have found that for my purposes, a sight is not necessary, so I did not resight the Winchester. I just use the end of the barrel itself. A gunsmith can reinstall a bead if you are concerned about the sight though. I've never asked a 'smith how much a barrel cutting would be, but it can't be much if you would like to just have it done.
edited to add: keep an eye on ebay for a bird barrel, and heed the advice regarding magazine capacities for migratory birds... it might actually be cheaper in the long run to buy an old side by side or O&U for the bird hunting..... See how they multiply?