Paul,

all materials have a certain elasticity. If you don't like the potato example, think of chewing gum. The point is that if you try to press an elevated profile into material of the same consistency, that you cannot drive it through that material without loosing its original shape. It gets displaced in all directions and ends up being crushed outright (for relatively inelastic materials) or 'swallowed' by the material it is pressed into. That makes it impossible to press rifling from the inside of a barrel to the outside, creating a 'rifling shadow' there, as some people tend to believe. These are stretch marks in a barrel that has been exposed to too much pressure.

Your idea about a mouse in a tube is correct, by the way. Barrels are elastic and always bulge / stretch somewhat when a bullet is launched through them. It is of course a dynamic process, but for the sake of illustration, imagine it like this: Take a bullet of relatively hard consistency, that is also, because of sloppy quality control, oversize. That increases its initial resistance to engraving and also its friction coefficient. Normally, if all is well, the bullets expands a bit in diameter by being pushed against it own inertia and still seals the barrel even though the barrel itself is expanding. That is what obturation really means (latin: Obturare = to seal).

By the way, if you have ever wondered why certain bullets shoot well with some rifles, and not with others - it is because there is a mismatch in the bullet's material relative to the rifle's material, disturbing this beneficial process. Happens most often with copper bullets, as rifle manufacturers (especially in older days) have made barrel steel that is meant to harmonize with soft bullets.

But back to the hard, oversized bullet. Imagine it getting stuck briefly - what happens then: As the bullet cannot move, the burning powder experiences a severe pressure spike. If the spike is steep (fast) enough, the barrel bursts before the bullet can be moved sufficiently to relive pressure. If however, the pressure spike is below a certain threshold, the expanding gas causes a bulge behind the bullet, which, because of the shape of a barrel, extends towards the sides of the stuck bullet, 'freeing' it and launching it forward. This may have saved your life as the barrel did not rupture outright, but could have exceeded the elasticity of the barrel, leading to exactly the symptoms that people call OSR. The misconception about OSR is not that a hard bullet is problematic (it is, mono-metal bullets as well as steel-jacketed bullets are more prone to pressure spikes). The misconception is that the bullet causes the damage DIRECTLY, while it is in reality excessive pressure that bulges the barrel beyond its point of elasticity. Even if a bullet were so much harder then the barrel that it could theoretically 'smear' out the rifling, this would never happen as the resistance by the rifling would be so strong that the pressure would go through the roof and blow up the gun.