Remington 870's
AdamGrier,
The only 21" barrel that I can think of for an 870 would be a rifled slug barrel. Although it would be possible to kill a turkey with this barrel, it isn't practical (your effective range would not be very far). If you want to buy a good all around shotgun and you don't want to have to take out a loan, I recommend a Rem 870 or a Mossberg, 26" or 28" barrel with screw in chokes, chambered for 3" or 3.5" shells. Whichever brand you choose, the screw in chokes will offer you more versatility and allow you to be able to do more than just hunt turkeys. Pattern your gun with different brands of ammo and know your maximum effective killing range and you'll be fine. I shoot a Rem 870, 3", Hastings Barrel with an Extra full Turkey choke, Remington Golden Premier copper plated #4's and my effective range is 50 yards. I am planning on trying “Hevi-Shot” this year but I will have to drop back to a full choke. Most of the older "Extra full Turkey chokes" are made for lead shot only.
Good Luck and Happy Hunting