One of the neat things about the 870 is the fact it has a rifled barrel for shooting slugs, and a smooth bore barrel for shot shells.
It takes about 5 minutes to change out barrels, this instructable will show you how.
Remember to follow proper firearm handling procedures when doing this project, and wear safety glasses!
Step 1: Ensure the Firearm Is Unloaded
Make sure the firearm is unloaded by opening the action.
Keep the action open for this entire procedure.
Step 2: Removing the Barrel Lock
Unscrew the barrel lock screw with you hands by turning it counter clockwise.
It should removed from the magazine area.
Step 3: Remove the Barrel
Slight up on the barrel until it clears the magazine tube.
Once off set this barrel aside or store in a safe place.
Step 4: Attach the New Barrel
Slide the shot barrel down onto the magazine area. Line up the base (chamber) area of the barrel with the action of the stock as you slide it down.
Step 5: Screw on the Barrel Locking Cap
Re-screw on the cap on top of the barrel connection to the magazine slide.
It is a good idea to also check the choke, to ensure it is still tightly screwed in when you make the barrel swap.
You are now complete, enjoy the shooting range!