What is the value of a 1975 Magnum Browning Belgium auto 5 shotgun?

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. Condition and age is also a factor. Auto 5's with recoil pads (other than Mag 12's) poly-chokes and compensators will reduce value. Recommend taking it to a gun shop for appraisal.

You can also do your own investigation by looking at other models being sold that are in the same condition as yours. Try the gunbroker.com or a periodical called "The Gun List"

You can also reference the Blue Book of Gun Values which can be purchased on line at various retailers