That's the point. If it's so hard that it won't obturate below 45,000 PSI, it isn't soft at all. Note that the maximum operating pressures of ALL flanged nitro express cartridges over .375 (large bore double rifle cartridges) are below 45,000 PSI, excepting the .500/.416. The inability to obturate is the danger to DRs. Further, the Barnes solid and TSX are not of true driving band design. The full diameter "bands" are far too wide to permit the necessary compression.

The two common forms of damage have been noted above - failed solder joints and OSR. I've seen both, OSR being the most common. Failed solder joints are self explanatory. The constant excessive expansion and contraction of the barrel metal due to the repeated passage of non-compressible bullets can cause perfectly good rib and muzzle wedge solder joints to fail.

OSR is a well known problem with mono-metal bullets in doubles that, for some reason I can't understand, seems to be very difficult for many to see, even when it's severe. It's caused by a non-compressible bullet too hard for the barrel passing down the bore, forcing the lands outward. The displaced metal has to go somewhere - to the outside of the barrels. The easiest way to detect it is to point the rifle at a light source with a straight edge next to it (a window pane is usually perfect) and look for the spirals (the reverse "imprint" of the rifling) pushed through to the outside of the barrels. This type of damage is common, and it isn't repairable. I can truthfully state that I've seen dozens of examples.

Gregor Woods mentions that in extreme cases of OSR, the barrels can actually end up bent. I can't be certain, but I think I've seen this once. The barrels were sucessfully straightened.

Steel jacketed solids have been available in factory ammuntion for double rifles, and in wide use in them, for almost 60 years. OSR doesn't seem to have become a regular issue in DRs until mono-metal bullets began to take hold in the '80s. I've shot up a lot of old steel Kynoch in a variety of rifles, including a batch of steel jacketed Kynoch SOFTS in .470 (before I knew how rare they were), and plenty of Woodleigh solids. I've never observed a problem of any kind. I've also shot a very few monos in doubles on two occasions, and got damage both times, in one case severe. The latter was with Barnes X, a total of less than 50 rounds (less than 25 rounds through each tube).

At flanged nitro express DR velocities (2000-2100 fps), I don't see any advantage monos offer, let alone enough to justify the risk.