Gerrygoat: I agree with you enthusiastically in your contention that the 6mm Remington should have been much more popular and long lived than it has been.
I was just reading this week (I forget in which publication!) that the 6mm Remington gives much longer barrel life than the 243 Winchester due to its case design/longer neck etc.
I believe that the article was in the latest issue of Varmint Hunter Magazine now that I think about it - in an article on internal ballistics IIRC.
I once owned a Remington 40X Rangemaster in 6mm Remington - it was a dandy and shot VERY well.
I had it a year or two when along came a chance to buy an exact same Rifle that was new but in caliber 243 Winchester - in my infinite wisdom of the time (1975-ish) I thought to myself well I can sell the 6mm 40X to a friend who wants it and add about $75.00 more to get the new 243 40X and I will have longer barrel life for my then extensive Prairie Dog Hunting.
So I made the deals - back then I did not have the discretionary funds that I have now.
I wish though that I had kept that 6mm Remington 40X as well!
I still have the 243 40X and it still shoots quite well but
I often think about that 6mm 40X and wish I had not sold it.
Indeed the 6mm Remington is a fine cartridge.
Hold into the wind
VarmintGuy