First thing, purposely putting an obstruction in a barrel and firing it is not only extremely dangerous but extremely stupid unless it's being done in a fully controlled and contained test facility.
Yes, Jim, physics does play a roll in all of this but not in the manner you are suggesting. A bullet encountering an obstruction in the bore is completely different than a bullet encountering an obstruction to its flight path outside of the bore.
Inside the bore, the bullet is contained by the bore, this is what allows softer bullets to be obtruded by the sudden pressure rise to fill the bore completely, the bullet obtrudes only until it is fully contained by the bore or until it has reached its maximum allowable deformation limit. Thus is why a bore with a loose or tight spot will often shoot well with soft-cast bullets but will be inaccurate with jacketed or hard-cast bullets - the softer the bullet, the more readily it will conform to the changing diameter of the bore preventing gas-cutting of the bullet and/or bore.
Because the bullet is contained by the bore, when it reaches an obstruction, the bullet will deform only to the point the bore will allow it to reach, once the maximum deformation has been reached velocity is rapidly lost - the rapid loss of velocity is what causes the rapid rise in pressure behind the projectile - it is the pressure rise that destroys the barrel if the yield strength of the of the barrel is exceeded. Barrels can only stretch so far and if you have enough gas volume and pressure to exceed the amount that the barrel can contain, the barrel is going to fail. If you could make the containment (barrel) strong enough, one of three things will happen, either the projectile and obstruction will be ejected from the muzzle, the gas pressure will be contained or vent slowly through leakage or it'll find the weakest point and vent catastrophically.
Because the projectile is contained in the bore, it will not be subject to the same amount of energy transition into heat as when it's deformed outside of the bore. Yes, there will be some energy transition into heat but nowhere near enough to liquefy the bullet let alone the bore with a considerably higher melting point. Barrel failures caused by an obstruction are caused by pressure because the amount of heat generated by the energy transfer is both too small and too slow to be of any consequence.
When a projectile encounters an obstruction outside the bore, such as a steel plate, there is no longer any containment of the bullet or obstruction material thus allowing both to be subjected to deformation. With the exception of certain extremely high velocity and specially constructed projectiles, actual "liquid" deformation is NOT what happens. In the case of common expanding jacketed or all-lead bullet, what happens is called "plasticized" deformation or flow. While the results may have the visual appearance of liquefied flow, what you're seeing is the results of cold plasticized transfer. Yes, there is what seems to be a large amount of heat generated but in reality the amount of energy transformed into heat is insufficient to cause completely melting of the lead let alone the copper jacket or steel. The spatters of lead attached to adjacent areas/objects result from cold mechanical transfer - there is heat involved but not in sufficient amounts to cause an actual bonding like happens with a solder joint but it is rather a mechanical attachment. Yes, it may have the appearance of liquid flow but it is not a liquid flow but a plastic flow where the metal is fully deformed but at a temperature below its melting point. Even when you do get a deep crater or complete penetration of the steel plate, the flow of the steel results from mechanical deformation not melting of the alloy.
Since Nick brought up localized ringing, I'll go a little further into that too because there are two causes of ringing - One is caused by rapid pressure rises resulting by the bullet intermittently stopping and the other is caused by uneven combustion of the powder or what's known as "SPS" (Secondary Pressure Spike". Ringing in or around the chamber is often caused by the bullet being ejected fully or partially from the case by the ignition of the primer, the bullet remains in its new position until the powder burn then builds enough pressure to start the bullet moving again. The increased volume of powder burn area caused by the movement of the bullet allows the powder ignition to happen in a localized area often resulting in a ring at the point of ignition, in other cases the excessive pressure rise caused by the stoppage of the bullet the weakest/thinnest point of the containment will suffer permanent deformation but not to the point of catastrophic failure. Ringing caused by SPS can happen at any point in the bore and bores subjected to repeated SPS conditions will often have multiple rings along most or all of its length - this is often referred to as "wash-boarding" because bores that are severely damaged by SPS will have the appearance of a rippled wash-board.
"Carry the battle to them. Don't let them bring it to you. Put them on the defensive and don't ever apologize for anything."
Harry S. Truman