For muscle memory and lots of trigger time, .22 LR is great. The obvious problem is that the recoil is very different and this serverly affects the controlability. I had the opportunity to shoot a Tommy gun in .45 ACP and an H&K MP5 in 10mm last weekend. I consider myself a decent shot with my pistols shooting slowly but full auto is a whole different ball game. My first shot hit what I was aiming at but the recoil opened up the groups quite a bit. The MP5 is an excellent weapon and has a gas operated recoil system, but even 2 round bursts were a bit hard to control. I'm sure a .22 LR full auto would be much easier to control but I wouldn't learn how to manage the recoil. I think it might even create a false sense of security if the shooter becomes accustomed to the .22 LR. I guess as long as the shooter spends a significant amount of time shooting the gun in the real calibers, .22 LR can only help.
The ATF should be a convenience store instead of a government agency!