From all of the details provided, my opinion is that a bulge due to shot bridging would be unlikely in an improved cylinder choke. Problems due to steel shot bridging seem to occur mostly in tighter choke constrictions. And I agree that a bit of light fluffy snow would most likely get blown out ahead of the shot charge. But snow is a relative term. ranging from light powder to almost slush. When you said the bottom barrel is choked "improved", I am assuming here that you meant improved cylinder.
But if you got some snow in a warm barrel right after you had fired a previous shot, it could have melted, and then froze, leaving a depost of ice at the bottom of the tube. Ice will not get blown out, and the wad and shot charge would have to displace it, and the barrel wall, to exit. That might also explain why the bulge is at the bottom of the tube instead of being more circumferal.
Bulges are much more difficult to repair than dents, but if it is small and not too close to a rib, a good gunsmith might be able to iron it down to make it less obtrusive.
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