When the word "patterns" is used, it is generally referring to "shotgun", and is not synonomous with "1 hole groups". Regarding your questions on the various cartridges out there that fire 6mm bullets, you will find a bunch of info on this site. It is probably the best site I have seen that really digs into the 6br case and its' offspring. Just read through the prior posts, and look at the different cartridge designs. You will see the pro's and con's and capabilities of the various designs. Hopefully you'll be inspired to shoot one of these exceptional cartridges.
Generally speaking, the older actions that you have mentioned are not up to snuff with custom benchrest actions, or even modified current factory actions. There is nothing wrong with older actions, and some can be tuned up and modified to deliver excellent accuracy. They just can't compete day in and day out in the extreme accuracy game with the custom actions that are made to exact tolerances with modern computerized machining tools and the best metals available. Most of the guys using this board are using custom actions and barrels that are capable of world record groups every time they are fired. It's just the wind and shooter variables that end up messing the groups up. Apparently, we all mess up quite a bit, or we'd all hold world records! The equipment being used is really that good. There is a big difference between 1 hole groups that measure .10 and 1 hole groups that measure .30 at 100 yards. Do not fret,,,,,,,,read up, hold off on throwing cash at an older action, and think about the next gun you want built!