The various Rem 870's are well illustrated on their website. Some companies sell various after market parts, but I believe the factory rifled barrel job is 20". The 18" barrels are cylinder bore buckshot tubes. (Pretty crappy for trying to shoot slugs accurately.) The difference in prices, I believe, are due to differences in stocks and finish. Barrel, action and tube are made of the same stuff. I was looking at a police model over $400 and it was because of the finish. I don't think slugs are made in shells over 3". (Anyone more up to date on this, please fill in.) Wish you luck with the forend grip fit! But one shot will probably drop your buck! Don't know about the Mossbergs. Used the 870 alot in corrections and military. Really liked it.