I think Mr Merrington would be qualified to judge what causes such small bulges. Since I was able to examine a set of 21 barrels currently with him for repair I can testify I have seen the UFO
. Possible I could get some pictures. My understanding as to how this can happen is as follows. In fact I was shooting quail today over my dogs with my sxs. I shot the right barrel followed by shooting the left barrel only a few hundred's of a second apart. When hunting I have done this many times. I have never experienced an issue doing this. I am guessing that many of us do this regularly. I am not under the impression that pimple bulges are an epidemic but that it can happen.The theory is that upon firing the right barrel the shotgun recoils. The shotgun is stopped from recoiling as far as it would if your shoulder did not stop. This reaction forces some of the shot which may not be properly restrained (the crimp) to be thrust down the left barrel out of the loose crimped shell at a much slower speed then the pellets that are soon to follow at about 1100 fps., when the left barrel is fired. I think that whether the barrels are pointed down, up, or level this could still be an issue. Some pellets may in fact clear the muzzle, some just may not.