I picked up a used police shotgun at a show several years ago.  It's Rem 870 Magnum with a 20 inch barrel.  I refinished the stock, had the parts reparked and bought a mag extension for it.  I see most tactical shotguns have 18.5 in barrel these days.  Is my gun not a police weapon or were barrels just a little longer a while ago?

Remington has made both lengths and as far as I know still does. The 18in. is slightly shorter than some mag extensions, and the 20" gives a bit more velocity, but not much. I would see if i could trade the 20 for an 18 if I wanted a shorter one, but cutting it down should be no great problem for a decent 'smith.