My 390/3901 barrel feels loose after cleaning.

390/3901 loose barrel

Caution: Refer to the owners manual and make sure the firearm is unloaded before performing any procedure.
The most common cause of this issue is the improper re-assembly of the barrel and gas system.  The following are instructions for proper installation:
# 1 - Place the ribbed portion of the gas piston into the gas piston housing of the barrel.
# 2 - Install the barrel on the receiver.
# 3 - Place the gas spring with the washer over the threaded post, the washer should cover the 4 holes on the barrel cylinder.
# 4 - Place the fore-end over the magazine tube making sure it is properly seated at the rear of the receiver.  The end of the gas spring should now be exposed through the front/top of the fore-end.
# 5 - Screw the fore-end cap onto the threaded post and secure it.
# 6 - There should be no more that 1/16" gap between the receiver and the barrel rib with no movement in the barrel.
You can also download the attached pdf that explains the proper assembly for the 390/3901 shotgun.