The barrel limit is 18", but the entire firearm must maintain an overall length of...I can't remember. As long as you're within those two dimensions, you're good to go.
Already have a 28 in barrel w/rem chokes for recreation. Have a class a machinest w/ his own shop for a cusin. Now that is econical. Besides the M/11 is for skeet.
You will lose the choke entirely. Short smooth bore barrels are relatively cheap (around $100+/-). You will maintain a choke and have a 100% legal no worries, no bare metal, fully finished with sight bead barrel.
You could also sell the 30" Bbl and simply buy a slug bbl. That'll give you some choke (IC) and sights. With all the 870 owners here, I would think that selling that 30" bbl would be easy. It should be worth $150-175. Slug bbls and police bbls (bead, no sights) are a dime a dozen, so you should come out ahead on that sort of deal.
If you cut your bbl, be aware that you will render it worthless for any future resale. If you cut your bbl you may not get the exact length you are looking for because you need to make the cut through the center of a rib post. Some concerns should be how to cut it without popping the rib loose. If it does come loose you will have to silver solder it and then refinish the bbl or put up with a touch up cold blue. All in all, it's not worth cutting that bbl, especially due to the fact that there are so many 870 bbls out there.
Think about this before breaking out that hack saw.
The whole gun needs refineshed and new furiture. I got second hand for 100 bucks and baught an all purpose barrel. Im after function not form. You use a lathe to cut barrels not a hack saw. This aint no pork chop opperation.