Yeah, I like 6mm. I got a caliber 6mm Remington rifle but could not get that thing to shoot, even after much load experimenting, so I sold it and got the cheaper Model 70, which was called the Model 670, in .243 and about a 10" twist.
That one shot OK if you let the barrel cool off between shots. You could see a consistent pattern of the shots moving --I think it was to the left and up --as the gun heated.
Apparently they weren't too careful about barrels in this cheaper model and they had to straighten the barrel after boring it, so it left stresses which showed up as the barrel warmed up. This was OK for calling, but when I used it for prairie dogs, sometimes the shooting was too rapid to let the barrel cool, and that was a royal pain. I often wondered if cryogenic heat-treatment would have relieved those stresses.
In addition, it was difficult to develop a load which didn't ricochet too badly at the flat angles of prairie dog shooting. The worst ricochets were from Zipedo solid-based bullets in 75 grain.
Anyhow, I gave the rifle to my son and he got it to shoot better by free-floating the barrel and using heavier bullets, but it still walks the shots, just not as badly.
Then I went to an ancient Savage rifle in .223 Remington with a 14" twist and was eminently satisfied with the 3/8" groups it shot, even though it wasn't the typical accuracy-nut configuration. Only one bolt lug, split receiver, a barrel band on the forend, not glass-bedded, with a sidewinder scope mount, but man, could that thing shoot.
And recoil was negligible. I ultimately replaced the scope that came with it with a Redfield 12X silhouette scope I bought from the factory tore here in Denver, and I put a Harris bipod on it. That rig was deadly out to 250 yards on those teeny tiny little varmints, and darned dangerous on them out to 300-350 yards, but I quit shooting at them that far away. And with the light Hornady SX bullets in 53 grains, I
never repeat
never heard a ricochet. Those bullets would absolutely disintegrate on hitting just about anything.
The .243 was touted as being an "all around cartridge," suitable for everything from varmints to deer, and indeed it was. And as I grew older, it got to be the biggest rifle I could fire comfortably and still be fun.
But their definition of varmint was a ittle different from mine, which included a little 3" wide and 8" tall prairie dog. I think they were talking about bigger varmints like coyotes and fat groundhogs. However, that Model 670 would have been OK for prairie dogs if it could use lighter bullets and they hadn't had to straighten that barrel out after boring. I figure a 12" twist would have been ideal for my purposes.
I guess nowadays, with better manufacturing techniques like hammer forging barrels on a reverse-rifled mandrel, barrel warping is not much of a problem any more, especially since the hammering would tend to relieve any stresses.
Pic: me at my favorite Prairie Dog Paradise with that rifle and scope, but a homemade bipod instead of the Harris. (The springs on the Harris go "blong" with each shot, which is transmitted directly to your cheekbone, and was moderately annoying untll I got used to it.)

Terry, 230RN