I would build a 243 Win since it is a great 6mm. It has been held back for decades because factories that mass produce rifles have never given it a faster twist barrel.
I will not build stupid cartridges that have no reason to exist other than they are new when older non-proprietary cartridges exist that are either as good or better than the newer cartridge. More companies make brass including match grade brass in 308 blank brass or 243 brass and normal brass for 243.
I and my oldest son have killed all kinds of game with 243 Win with both stock rifles and with custom fast twist barreled rifles. Millions of people have killed everything from Elk to Coyote with 243 Win. Not sure millions of people own a 6CM and even less sure that millions have killed. A lot of people have also used 243 Win for F-Class, Silhouette and NRA Hipower X-Course including David Tubb.
Some especialy like the 243 AI and have shot inside the 6.5 and 7mm cartridges in F-Class.
I am hoping some idiot will trim CM case so it holds just a few more grains than the 30-30 Win and will make a 30-30CM so I can sit back and laugh at all the people that pop up wanting to build 30-30CM to gain 50fps or even 100fps and pay 3X as much for ammo and brass from just one supplier of mediocre brass with no significant increase in down range killing effect on actual game animals at actual ranges practical for the bullet and cartridge combo!
I am going to let this top secret info out of the bag since we are discussing CM cartridges. On ballistic Gelatin with the same bullets used and the same brand of ammo used there is no statically significant difference in performance between 6.5CM and 6.5PRC. On game animals no significant difference on game animals between the 6.5CM and the 6.5PRC. On paper the 6.5PRC does put up better numbers but on game and ballistic gel they perform the same.
This also explains why we do not see any enhanced lethality between the 6.5x55, 260 Rem, 6.5CM or 6.5PRC numbers on paper do not always reflect performance on game! Likewise 6CM has not produced any better field results than the 243 Win and given the same barrels and twist rates both will perform so close to identically that it is not even a contest it is 6 of one and half a dozen of another. The only place it would matter is for those that do not hand load and those with out custom built rifles.
It is hard to find any brass right now or loaded ammo but I have seen and have bought 243 ammo over the Pandemic. I have seen and purchased 6.5CM ammo as well and 30-06, 308 Win and 45-70. No place I shop has had 6CM ammo or brass that I have seen during the pandemic.
1. Still is.
2. Judgement of a thing as stupid in a blanket statement which ignores any nuance is probably the judgement failure. That's making a conscious decision to not use something that performs identically to something else while doing so more efficiently.
3. Interesting that you'd drag David into this. There's a particular reason that he uses 6XC. It takes 5gr less powder to do the same job which it does with better brass and barrel life.
4. Argument from popularity is no argument at all especially when your own (#3) examples throw shade at your statement. 243win is a shitty case design because it wasn't designed it was adapted, badly, from an existing poor case design meant to do a specific job where compromises had to be made to sustain fully automatic fire. 243 win suffers greatly from its tapered case and shallow shoulder angle in terms of brass growth, bore erosion and generally crap efficiency upside a .243AI or 6SLR or 6CM. Just because not very good was good enough for generations doesn't make it good enough now.
5. Hoping for people to behave in a way you can laugh at them for is pretty childish and a symptom of low self-esteem.
6. There's nothing top secret about the fact that 6.5's don't really deliver much different performance across a wide range of velocities. It's been known for literally 50 years. Just not many people are able to move past racing paper for fun and profit.
7. Redundant paragraph is redundant.
8. Waving hands around about what's odd during an odd situation doesn't speak of very clear thinking. That said, it was easier to find 6CM over the past 2 years than it was 243win probably because it's not as common as a hunting cartridge and it is WILDLY more common in match use where handloading is the norm in which case brass and loaded ammo are not likely to be common anyway.
Contradicting arguments nobody is making is called a straw man fallacy. That can be excused as emotionality. Not making a fact based point that holds up to the littlest bit of scrutiny, that's a little harder to excuse. Contradicting your own argument (the blurb about .243AI), well that's just sloppy thinking.
factory 6CM ammo has remained readily available and priced well below 243 ammo (when you could find it) during the lastest ammo craze. 
