[Jack]

On the 243, Vs. 6mm Rem issue....
Winchester DID do a good job of marketing the cartridge as a combo deer/varmint rifle. And chambering the 243 in the Model 70 Featherweight sure didn't hurt.
And, Remington helped them, a LOT.
The tale of twist rates is well known: the 243 a 1-10 twist so that it could handle the 100 grain loads, and Rem using a 1-12 twist in the 244 Rem, in theory for better accuracy with light bullets.
Remington viewed the 244 Remington as a varmint cartridge, pure and simple.
Winchester, from the start, loaded 80 grain varmint loads and 100 grain deer loads- and marketed them as such.
Remington's factory loads when the 244 came out were 75 grain and 90 grain- and they weren't marketed as deer loads.
The 6MM is a really fine cartridge. But, by the time Remington wised up to the dual use (varmint/deer) concept, the 243 was way out in front, and chambered in a lot of brands of rifles.
The problem was never the cartridge, really, it was Remington's marketing.