Thread: overlap between cartridges

This particular one has come up a few times as of late and a discussion with fellow forumite this morning really brought it to the fore.

with the advent of fast twist barrels in some common cartridges allowing much heavier projectiles to be used they almost jump up a bracket in whats expected of them...in particular the fast twist .223 using projectiles 65-80grains.... thats getting into .243 territory all be it at slower speed..... and where does that leave the likes of the 6x45???? dead in the water some might say
there is little the fatter bore can do the skinnier one cant
same argument has gone on forever.
.308w with a 110grn pill for reduced recoil = a .243 pretty jolly nearly and a .270w loaded with 100-110grn projectiles is in same territory but uses more powder to achieve the same thing.and the 25-06 too to some degree
you can always download a bigger cartridge to do what a smaller one will do,but cant always crank smaller one up to match the big boys. the 6.5 thread touched on this rather well
so I guess my point is this

WHY would you now want to rebarrel to something like the 6x45 when you can do pretty much the same thing with a std fast twist .223 with a LOT less mucking around and still have option to run off the shelf ammunition???
other than the why my dog licks its bits answer,I cant think of one
there have been many cartridges created in last few years to run through the AR platform..but if you remove that platform from equasion the cartridge offers sweet fack all that the run of the mill,older cartridges dont already have covered,and if going down custom or rebarrel route where twist rates can be tweaked,the older jobbies arguably stack up better still
with the push for monolithic projectiles we may see need in future for light for calibre to get velocity up..... but same deal still applies,the bigger /older case may do it better.
ramble over,interested to hear your thoughts and experiences