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I suggest you gents read Graeme Wright's book shooting the English double rifle. I'll try & summarise a bit without waffling on.
Page 226 refers to "hard projectiles", these Graeme refers to as 'any projectile other than a conventional lead cored projectile with a gilding metal or copper jacket'....'The major difference is that all these materials are much harder than lead. Consequently, it requires that much more force to extrude and displace the projectile material away from the rifling and push the bullet through the barrel. One possible non-lead exception to this definition are projectiles using driving bands'....
'However all will be more stressful than a conventional projectile with its soft lead core and relatively thin, soft jacket material'.
Graeme then refers to an experiment the Geoff MacDonald of Woodleigh bullets did as an experiment to a section of .375" barrel (of correct bore & groove diameters)turned down to 0.090".
A soft point was forced through the barrel with a hydraulic ram, and measurements were taken with a micrometer.
'When forcing a soft point through, the barrel has no measurable expansion at all. However a steel solid gave 0.0005" expansion to the barrel, although the barrel did spring back after the bullet had passed'.
On P227 he refers to how the old Kynoch solids were made, with much thinner side walls & tapering projectiles - the photo shows it all very succinctly.
And so it now becomes obvious why these "old" solids fishtailed & lost track sometimes!

The one interesting note he gives is this as a final note on hard projectiles:-
'On a practical level it is interesting to note none of the current London Gunmakers recommends hard projectiles in their double rifles and in some cases single barrelled rifles. Speaking to the regulators themselves, they only use soft point ammunition during the regulating process. Only at the final stage do they fire a few solids just to check that the two projectile types have the same point of impact'.

Graeme then discusses barrel steel and barrel size, in which he tells of some cases that he knows of & how they got around certain problems - but not using hard projectiles!
He also notes that he has measured solids from a variety of bullet makers, in most cases he has found them often 0.0005 - 0.001" undersized, so will significantly reduce the stress on the rifling and hence pressure overall on a barrel.
He also discusses Rifling type, the current situation (rifling dimensions & bullet manufacturing), barrel splits, slow powders, Flaws or faults & Bore and groove diameters before doing another note on hard projectile before his conclusion. And I quote:-
'Firstly I am in contact with double rifle shooters all over the world. The vast majority (myself included) use standard lead cored soft points for nearly all their shooting. Typically even the use of lead cored solids is kept to a minimum,...'
'The second point is related, but deals with wear and tear. If new barrels were subjected to a lot of use with hard projectiles will they show signs of stress in the future? I guess only time will tell'.
His recommendations or safe guards are on P235 & a footnote P 236 in regard to Geoff MacDonald tests on barrel steel hardness and what he recommends in regard to age of rifle as to its suitability or not to use hard projectiles.
get the book fellas - its a wealth of knowledge & has reloading data as well!
Cheers
93x64mm

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