We know because the manufacturer has already stated what the RCMs will do, and others have also tested them. In a nutshell, they do the same thing as a number of other cartridges, but in slightly shorter barrels, in a shorter actions, with ammo available from only one manufacturer (so far) and in rifles available from only one manufacturer (so far).
Misunderstanding here. I was talking about the actual amount of time that it takes something to be accepted. The manufacturer's in France published, after the secret was out, the specs on their new powder, their new bullets, and their new rifles. It still took years to be accepted. That a proprietary cartridge, or family, is available is not something that's rare. Look at Weatherby, or Dakota, or Lazzeroni.
I can't agree with you here. Hunters and shooters try out and adopt new stuff every day. Stop and think about all the things we have now that our fathers and grandfathers didn't have. Composite stocks are everywhere now, as are stainless steel firearms. Laser rangefinders are commonplace. Variable power scopes have been around a while, but I am pretty sure your great-grandfather didn't have one. All of the above items represent progress in one form or another. Hunters and shooters have adopted new cartridges in the same way, when they offered something truly unique or progressive. The 17HMR comes to mind.
Composite, and laminated stocks have been available for over 60 years, as the Germans used them, and the Russians, in WWII. Yet, it wasn't until the 1990's that they made it mainstream. Stainless steel firearms are another of those incremental steps I spoke of. They made absolutely no difference in the design of the existing weapon. Laser rangefinders are commonplace, yes, but not those combined in the scope, which is what I spoke of. Yes, my great-grand-father had variable power scopes, before WWII. That's a while ago. Yet, they really caught on only in the 1970's, and then only in 3-9 power.
Again, what you discuss has been no more than incremental steps upon existing designs. We had range-finders, while not lasers, in the 1930's. As for the .17 HMR, necking an existing case down to hold a smaller diameter bullet is a trick from the early 1900's. As for new, it's simply a rim-fire version of the existing .17 Remington, not exactly a new round, at all.
Hunters didn't flock to the 5mm Remington Rim-fire, even though it was clearly far superior to the .22 cal. rim-fires, both long-rifle and Magnum. There have been literally hundreds of calibers brought to market, not because they were different, but because they were the company's OWN. The difference between the .270, and the .280, for example. Yet, thousands of rifles were sold, and people argue today, over a difference that exists mostly in the mind of an advertising company.
All I'm saying is that these cartridges, and the companies that make them, deserve less nay-saying, and knee-jerk reaction. It's their money, not ours, that's spent to bring new calibers, and rifles to market.