Found an interesting tip, from a cheapie Rossi 20 ga I bought 20 years ago that I'm passing on for info. The gun patterned nicely from the rt barrel, but the pattern from the left was an oval, skewed low and left. Looking at the muzzle, the left barrel was about 1/16" shorter in the low left side. I figured that the pressure was being released first there, while the gases still pushed on the upper rt side of the shot wad. Filed the muzzles square, by eyeball and a carpenter's square, problem SOLVED! Slugs were even going thru the paper sideways, low and left before. Probably why I got the thing so cheap. That and the birch stock, which I replaced with a piece of fiddleback walnut. So, not only the choke determines how the gun patterns, the squareness of the muzzle has a lot to do with it.
