Not picturing this...
You said that the mouth of the chamber measures smaller than the inside of the chamber. Unless I am mistaken (which frequently happens) it should be the other way around.
Most cylinders (I think probably all cylinders) are chamfered from the factory. The diameter at the very rim or mouth of the chamber should be slightly larger than the diameter of the chamber further into the chamber, You should see a notable chamfer in the chamber which extends into the chamber by about a sixteenth inch. I think the typical chamfer angle is 11 degrees but others on the forum will need to check me on that number.
When I bought my first mold for my .31s I went by the recommendations in the various catalogs and selected a .315 diameter round ball mold. Bullet was too small and did not seat well in the chambers. I had the same problem you are having in that the bullets in the non-battery chambers were moving forward upon discharge. I went to a larger mold (I think it is .320) and solved the problem.
I must hasten to add this note of extreme caution.
The seating of the bullet on the powder is established at loading. There can be no air gap between the bullet and the powder lest you risk failure of the cylinder upon discharge. If the bullet moves forward in the chamber you might assume that an air gap has been created. Very dangerous.
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