Assuming a lot. Iron,490lbs/cu.ft--4.5oz/cu.in,open circumference average (12 ga) --2in.; length --30 in.Then for each .001 in. removal--.06 cu.in or about .25 oz/barrel.So,to get 1 oz, both barrels need to lose .002 in. open area full length.That is if I kept all the decimals in the right place."Damm the decimal point" said the engineer as the bridge fell down.Chuck H is the cautious engineer,just get a different bridge.