I have a Rem 870 with an 18 inch barrel cylinder bore. Can this barrel be threaded to accept a screw in choke?
Yes, it can be threaded for Rem-choke. There are a lot of websites that offer these services, just do a search for "choke tube installation". A bit of advice, stay away from this one:
www.guncustomizing.com. I sent mine to him because he had the cheapest prices. I had to get the Better Buisness Bureau on him. He would not reply to my e-mails asking for a status, after I was twice told he would have it back to me in a couple of days(three weeks between each time). 2 Weeks later, when I gave him a call on his cell, he said he had been too busy to reply to my e-mail. I didn't get my original barrel back, but one identical to it--that is after I sent a complaint to the BBB.