It is more complicated than just lead vs. copper, and whether a boat tail improves BC whilst pushing the length forward for the same mass, or a compromise. Whilst having to engineer the thing and machine it to deliver good terminal performance for hunting across a wide range of use cases. Not easy, somewhat subjective, and still very much early days I think.
Or simplified theoretical stability calculation that knows nothing about the mass distribution of the material of the bullet, nor the aerodynamic instabilities of it's shape. Nor the precision with which it has been manufactured, no bullet will be perfectly in balance for spin stability, they will all have some degree of nutation.
Poorer ones might just diverge until they even end up flying sideways. Good ones might manage to converge. Sometimes giving better results at longer ranges ("going to sleep" theory. Target shooters have known this for years, trying to push lead cored stuff to the ragged edge. Whilst using the slowest twist rate that works, over-"stabilisation" with fast twists being noticeably detrimental to accuracy too.
Hence the received wisdom to go light for calibre, push velocity (for stability and possibly terminal ballistics, depending how "hard" the construction is), etc.
That seems to still be the thing with Barnes, but now it seems there are many others having a go at this, with their own ideas, some may be very good indeed, others less so, 'twas ever thus.
Then there is the alloy composition. Copper is not necessarily pure copper, I doubt that that would work very well at all, at least only for a few shots until the barrel was copper fouled.
Indeed with things like Fox I think they only claim to be some sort of brass, which is a substantially less dense material, being alloyed primarily with zinc.
So I'd say it is early days before non toxic materials are as well understood and proven as the old lead stuff, but it does seem that already we have some very good options.
However, I think it is pretty clear that a .243, originally designed to shoot say an explosive lead varmint bullet in the 70 grain class fast and flat, maybe nowadays with a faster twist taking it up to be usable with 100 grains, is becoming borderline for non toxic, unless say the Scots see sense and change their regulations to make more sense for what I'm sure could still be a very effective calibre, but with slightly lighter bullets.
It's all very odd. I can, and have legally shot Fallow and Red in England with .243 70 grain bullets (not very suitable explosive ones, estate rifle, normally used for park culling, headshots). Illegal in Scotland.
Yet I may not shoot a Roe with say a .223 or smaller. Whereas that is deemed a very suitable calibre in Scotland.
However I can shoot muntjac with such things, which In my limited experience are far far tougher than roe, which are delicate things by comparison it seems to me Even a classic boiler room shot with a 30-06 resulted in one runner, at least 150 yards, heart destroyed, one shoulder literally blown off (precise raking shot from the left side, unbelievable, only stopped when it had basically bled out. Adrenalin, reflexes, unshockability, determination I'm sure. Respect.
Of course, officially the are as yet no Muntjac in Scotland, and long may that continue, seriously, despite being great sport and very tasty. So their people might not have an opinion about what to shoot them with, and hopefully won't need to learn.
So we do have some inconsistencies in legislation which I think need to be addressed in anticipation of lead free, possibly changed, brought into line, or at least properly researched, because, I have no doubt that this is coming. Meanwhile .270, .308 etc. are safe bets and known to work.
So I am told.
Never having yet shot a "copper" bullet.
