Value is too difficult to be accurate without seeing the gun. Condition which includes the amount of original finish and original configuration will determine value. Auto 5's with poly-chokes, compensators and recoil pads (other than Mag 12's) will reduce value. Near mint guns and original box will command an extra premium. American A5's are usually 25-40% lower thantheir Belgian cousins.
The Citori is made in Japan.
I think you may have meant a "Browining Sweet 16" instead of "18", which is a Belgum-made Browning shotgun. The value of the gun depends on its condition. In good condition the value should range somewhere between $900 to $1,200. But depending on the area where you are, the value could be higher or lower.
I would need to know what model of shotgun we are talking about before I can say when your browning was made.
Browning never made a Model 51 shotgun.
AFAIK they never made a bolt action shotgun
1962.A Browning light 12 shotgun with the prefix of 3G indicates that your shotgun was made in the year 1963.
100-1000 usd
1943 browning light 12 belguim made $5K
Are we talking about a auto-5 shotgun,or a BPS(browning pump shotgun),or a over-under shotgun(citori, or superposed)These answers would help me determine the value of your shotgun which was made in 1980.
Need the guage to determine age
Minimal
100-10000 or more depending on model