

I agree with the comments on this forum (https://www.thehighroad.org/index.php?threads/mushrooming-22-lr-case-heritage-rough-rider.891807/) that the cylinder could have a machining defect. (Basically a headspace issue, also but kinda not. The cartridges are sitting too deep in the cylinder itself.)
If there is room for the brass to get pushed back far enough for it to mushroom out, something is seriously wrong. The issue could manifest from a machining error as small as 5-10 thou, I am speculating.
My first thought was excessive chamber pressure, but the bulging would be much worse around and behind the rim itself if that were the case.
The bullet and the brass get pushed in opposite directions and if the brass can move, it will move before it deforms. If it deforms, it’ll deform at the weakest spot first, like we see in your pic. (Excessive chamber pressure tends to expand the brass and lock it in place. With center-fire, it’ll blow the primers out or have a hole punched in them from the firing pin first. With rim fire, the pressure pushes back on the rim.)
Also, check for excessive slop with the cylinder. If it can move forwards and backwards too much, that could also telling of issues with other parts of the gun. (Like I mentioned before, it doesn’t take much for a gun to be out of spec enough to cause issues with brass.)
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