RE: 243 or 6mm for deer
The 6mm (or 244 rem) is a great heavey varmint cartridge. But it doesn't do so well with heavier bullets. As far as I know, there is no factory 6mm of over 100 grains. My 6mm shoots the 80 gr bullter beautifully but it shoots the 100 grainers very badly.
There are loads in 6mm over 100gr. I shoot 105gr remington extended range flat based. I've never seen a 6mm that wont handle a heavier bullet mine shoots the same with any bullet and i shoot in it from 80-105 accuracy is always great. You may have a 244.rem which doesnt like bullets heavier than 80gr due to the rate of twist.In the 6mm rem though they corrected that and it has no trouble handling a heavier bullet. Ifind it to me alot better on deer size game over the 243.If it was me i'd pick the 6mm over the 243 any day hands down.