Although this mystery has been solved to my satisfaction, for the sake of discussion (and for those who enjoy solving puzzles on their own) I'll withhold my solution and provide only some of the clues.
These two bullets are shown just as they were cut from the same well-used brick of Clear Ballistic gel. They came from the same lead pot, dropped from the same mould, and were fired with the same powder charge from the same .44 Bulldog snubby. The bullet on the left is the expected .43 caliber. The bullet on the right is closer to .40 caliber and now drops freely through the Bulldog barrel.
Finally, the 2 o'clock void in the bullet on the right is an embarrassment -- not a clue. No further comments on that are required.