Some years back I was asked to look at a Parker 16 ga that was missing many targets that seemed to be centered. The first thing we did was get out a patterning board, and found both barrels shooting laterally elongated patterns without much high or low. The muzzles bore that out, thin and oblong. The chokes had been opened by an unknown gunsmith to .006 or so, and there was not enough barrel wall thickness left at the muzzle to bore them again to round.
So the lesson is I suppose to be very careful, have enough wall thickness, ream them out gently, or better yet, leave them alone, use spreader loads, or just shoot for the head.
A reloader can get a greater spread by not using a shot cup, just an over powder wad. Or an even greater spread by a true spreader post and wad combination.
Keep in mind that the difference in a killing circle on a pattern board (30" at 40 yards) between full and cylinder is a 6" radius. So chokes give you inches in killing effectiveness whereas misses are usually much greater than that. Doing shotgun instruction, you look over the shoulder at the shot pattern by the clay and see misses by three feet or more. That is for an effective killing circle on game, generally six or more pellets on a game bird silhouette in the 30 inch circle. If you are shooting clays, one or two pellets is all it takes to break it. So, when you are considering chokes, keep in mind there is a difference between a clays effective choke and the tighter choke necessary for an effective and humane kill on a game bird.
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