It was made in 1975. Use the link below for value http://wiki.answers.com/Q/What_is_the_value_of_a_Browning_Sweet_Sixteen
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.
100-10000 or more depending on model
Minimal
Requires professional evaluation.
Your browning auto-5 shotgun was made by FN of belguim for browning in 1938.In the condition that you describe your shotgun should be worth between 450-550 dollars.
To determine the exact value of a Browning automatic 20 gauge shotgun made in Belgium, a couple factors would need to be taken into consideration. Some of these things are the age and condition of the shotgun.