As usual, the gunmakers misunderstand bullet design and thus classify all monos as being the same which they are most definitely not.

If you fire a Barnes X or banded solid down your double, it will slightly and temporarily bulge the barrel as the bullet travels down the barrel. This is because the hardness of the bullet material prevents it from obturating. The same is true of a Woodleigh lead-core steel lined solid, although they are not quite so bad. Shooting any of these bullets in a double of any age, but especially a vintage gun made form soft steel, should be minimized in my opinion. If you speak with the gentlemen who regulate newly-made British doubles, you will find that they regulate with Woodleigh SOFTS. Then they fire 2 Woodleigh solids to see where they hit and they are done. So even the regulators minimize use of very hard bullets like steel-lined solids.

A properly made monolithic bullet, such as the North Fork flat nose solid or the GS Custom flat nose solid, uses very thin driving bands to engage the rifling, but the shank of the bullet is smaller than the lands of the rifling. This allows the bullet to engrave on the rifling with very little pressure. And it is not necessary for the bullet to obturate because the driving bands easily deform into the grooves behind them. Thus the bullet does not bulge the barrel as it travels from one end to the other.

Contrast that to the Barnes banded solid and the Barnes triple shock bullets which have really thick bands and will still slightly bulge the barrel as the bullet travels down it. GS Custom and North Fork solids are actually much kinder to your double than Woodleigh steel-lined solids.

As for OSR, I have seen it.