So, here's a generalization for 12g double guns with about.035 wall barrels. Figure about 1 oz. per inch. Yours is a 16g and will be less. If you need half a pound off the gun, figure on cutting about 10 inches off.
As for them shooting as they did before you cut them, I can tell you all my 12g and 16g guns have converging barrels and they do not go parallel near the muzzles. I will put dollars to donuts that yours don't either. That means you can cut them and not have to re-lay the ribs or adjust the convergence.
Personally, I think cutting the barrels is not going to make this gun into what you want and certainly won't make it what most anyone else wants later if you decide to sell it. But it's your gun so hack away.
