Tinker,

your ideas about a hammer are useful. Let's first remember the main claim about OSR: 'A hard bullet presses the rifling from the inside to the outside, while a soft bullet does not do so'. Let's play 'Mythbusters' and set up the following experiment: Take a potato, cut it in half, and then carve a heart into it. The potato is the barrel, the heart represents the rifling. Now take a hammer (which represents the hard bullet) and violate the potato with some strikes. Now turn the potato around and look for the heart being visible on the other side. You won't find any pattern, irregardless of whether you strike the potato with a hammer or a bag of feathers. The reason is simple and shows the main fallacy of the OSR claim: No force or material on earth can drive a profile through a material that is of the same hardness and consistency as the profile itself. Take another cut potato and put a little wooden heart on it, hammer the wooden heart into the potato. Now turn the potato around and you will see the heart shape imprinted on the other side. Hence, OSR could only occur if grooves and lands were made of different materials with significanty different levels of hardness.