of the receiver.
There will be an oval with four number inside, should be one or the other of:
16-65 or 16-70.
the first number is the gauge and the second is the chamber length, either 65 mm's or 70mm's (2-9/16 or 2-3/4")
With the forward safety, you most likely have the 2-9/16 chamber.
Good news is there are 2-1/2" shell available.
Check out -
www.16ga.com - ther is a dated list of ammo suppliers.
Change out the recoil spring from around the mag tube, and the friction bronzes with new, as it should cycle like new.
ENJOY!