You will want to watch out for older models with the 20 gauge 870s, if you want to find/add extra barrels to your set. Barrels for the older large frame 20s are very difficult to find. The older 20 gauge 870s were made on the same size receiver as the 12 gauge. You can tell what size the receiver is by the receiver code, the last letter in the serial number.
Receiver codes on 20 ga. 870s are as listed below, the codes are the last letter in the serial number. Receivers left the factory set up for either 2 3/4" shells or 3" shells and are differentiated by the codes in that regard as well.
Full size frame, 2 3/4" = X ( began production in 1950)
Full size frame, 3" = N (began production in 1962)
Small frame LW 2 3/4" = K (began production in 1972)
Small frame LT-20 3" = U (began production in 1972)
Call Remington's toll free number, work your way through a couple of steps on their phone tree, and give them the model and serial # of your gun. They'll tell you what year it was made. Parts that are interchangeable among gauges are listed thus on the parts list as are parts specific to certain gauges.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.