Thanks guys for the formula and the discussion. I realize there is imprecision in estimating long forcing cones and length of choke. However, the formula worked well for gun I had back bored. It was a Day 500/Miroku. It had very heavy barrels around .050+ before work and was underbored .605 (a 20 gauge. After backboring about .021, my notes show it shed 4.3 ounces (28" bbls). The formula comes really close.I back bored it to rebalance the gun, after estimating the weight reduction with formula I lost. Now the gun weighs 6 pounds 3 ounces and is very well balanced! (I also drilled the stock a bit.)
I sent this information to my friend here in Anchorage who doubted the weight reduction.
Mike
