A Browning A5, in 3" 12g?
If my memory is correct (it's been 15 years), there are a two brass "friction rings" under the forend, over the magazine tube, between the spring and the barrel. There are also three bushings that are beveled on the inside edge to fit over the friction rings (the barrel fitting acts as the forth bushing).
Going from the rear of the shotgun, the order should be:
spring
bushing (beveled edge forward)
friction ring
bushing (beveled edge rearward)
bushing (beveled edge forward)
friction ring
barrel (which will bevel toward the rear)
These rings are compressed when the barrel cycles (as the barrel travels rearward). More rings gives more friction. For 3" loads you need the full set of rings. For low power loads (low brass), you need to remove a friction ring (and the two accompanying bushings).
-nosualc