I've used a hacksaw on a few. If you cut it where the rib posts are you have the option of cutting at 21" or 19" and still keeping it legal. I did the 1st one at 21". If I messed up I could take it to a pro and still have room to work with. I found that I liked 21" best and that is where I have cut the others.
You can buy these to replace the front sight. If you want to go cheap this works well.
HIVIZ ETA Front Sight Remington 870 1100 11-87 Vent Rib Fiber Optic 6 (midwayusa.com)
I did take one to a gunsmith and have him cut the barrel shorter, replace the bead, and rethread it for choke tubes. It cost me $150, which is cheaper than buying another barrel and I liked the versatility of still being able to change chokes.