Time flies. That is absolute truth.
Sadly, I can offer no absolute truth in the question of your duck gun. You are correct maybe, 9,999 times out of 10,000, the snow will get blown out of the end of the barrel. What happens the time it doesn’t, is usually destructive. Because it happened in front of the choke, and is not perfect in circumference, I’d lean toward something being in the tube, near the end, at the shot. It could have been debris from a previous shot, coupled with snow, but, it would appear it created a localized pressure spike that was able to deform the metal where the pressure developed by the shot is actually lowest, the end of the barrel.
I would hate looking at it, and the fact it happened. But, from all reports I have looked at, bulges at the business end are pretty much a cosmetic nuisance. The magic words here, might be “duck gun”. They are in perfect condition the first time out, after that, well, often, not so much.
I’d keep an eye on it. Measure it after each use with a micrometer, and commit the number to memory. It it gets worse, get a different gun. But, I would keep using it, until that happened.
Good luck.
Best,
Ted
