Reading the claims and counter-claims on the issue of OSR, I think it is important to take a step back from the various attempts of explanation.

Gun writers, first of all, have often only very cursory knowledge of internal ballistics. What they state on issues of this kind is usually what they have heard elsewhere, not necessarily something they either understand or have thought through.

Bullet makers have of course knowledge in their particular field, but arrive at conclusions about their products quite often by trial and error rather then by applying scientific theories. Consequently, they have a hard time explaining a particular process relating to their bullets. Usually, they will tell you they have done 'this and that' and therefore, in there opinion, there is no problem. 'This and that' is of course a test with regular barrels in this case, and I will get back to that, as it is important in regards to claimed observations of OSR.

In very simple terms, launching a bullet through a barrel means that a harder material (the barrel) forces its shape on a softer material (the bullet). As long as the material of the bullet is significantly softer then the barrel, and that is the case for traditional bullets as well as solids, then the barrel will displace bullet matter, and not the other way around. Irregardless of how hard that may be, this process is done with after the bullet has traveled the distance of its own length - it is engraved from this point on and does not offer further relevant resistance on its way along the barrel. That gets us to why bullet makers tend to undersize their solids, or why they apply drive-bands. A tougher bullet material is initially harder to displace, making loads that employ such bullets prone to early pressure spikes if not properly adjusted. Damage to a rifle from such a bullet may therefore occur as a result of excessive pressure that exceeds the tensile strength of the barrel. This effect, if it does not blow up the barrel outright, is of course felt along the entire length of the barrel while the bullet still travels inside. The prerequisite of a bursting barrel (provided it is the weakest link) is that it bulges and bulges until the barrel cannot take it anymore and bursts. A barrel that was just before bursting point because of excessive pressure should therefore look very much like the claimed manifestation of OSR. A barrel exposed to that kind of pressure would not spring back fully to what it was before, and in a double it might well have loosened the soldered rib. However, this displacement in the barrel was caused by excessive pressure as a result of the INITAL engraving resistance of a bullet that was launched with a load that was too strong for that type of bullet.

That brings us to why bullet makers claim these bullets are safe, while traditional rifle makers tend to advise against them. I might note here, that my modern H&H double was regulated with TTBC bullets, which are also mono-metal (in the shank area - and in .470 caliber only their shank is actually subject to engraving). The simple reason for this is that a modern gun, with the properly adjusted load, can shoot mono-metal solids without any problem. With the right load they do not even have to be undersized, and they could actually be a tad harder as well. The problem is that a lot of people don't think when they're reloading and just transfer load data without regard to bullets' differing friction and engraving properties.

Older guns, with generally thinner barrels and softer steel, are a therfore a different matter. They simply do not have the safety margin that modern guns have when a load is not adjusted to a particular bullet's properties. I might add here, that it is not always old manufacturing processes or the thin barrels that make them more prone to damage: There are gunsmiths that 'fix' off-face doubles by heating and bending the worn-out locking lugs. You can imagine what improperly applied heat can do to a rifle. According to my gunsmith this practice is more widespread then one might wish for, and quite dangerous for the shooter, as the metallurgical properties of the entire barrel-bundle are altered by this.