The .22 LR is designed so the case is the same diameter as the bullet. The bullet has a reduced-diameter base, which the case crimps into. The nominal bore diameter (to the bottom of the grooves) for a .22 LR is 0.223".
The .22 WMR is designed so that the case is larger in diameter than the bullet, and the bullet is seated much like a standard centerfire cartridge. This was done to improve accuracy (more consistent bullet tension). The nominal bore diameter (to the bottom of the grooves) for a .22 WMR is 0.224". This was done so that the option to use the plethora of widely-available .22 centerfire bullets would be open in the future. Since the case is larger in diameter than the bore, this means that the chamber is several thousandths larger than the bore.
Firing a .22 LR in a .22 WMR chamber means there is lots of clearance around the case. The cases will bulge, and sometimes split meaning the cartridge pressure isn't contained by the gasket of the cartridge case, which is what a cartridge case is for. You have a good chance of having hot gas, powder grains, and brass fragments squirting back out of the chamber at a very high velocity. Think what that will do to your eyeball.
This is why .22 LR/.22 WMR guns (like the Ruger Single Six) have separate cylinders... so they can have chambers properly sized for each cartridge.
Just because you can do it doesn't mean it's a good idea. When I was young, I got the bright idea of hitting a .22 LR cartridge with a hammer while it lay on a sidewalk. (I got this bright idea from watching an episode of "The Avengers" where Patrick McGoohan shot an enemy spy across a pasture by putting a cartridge in a fence knothole and whacking it with his cane. The 100 yard running shot he made was truly fantastic, and inspired my imagination. Who needs a gun when you have a fence and knothole, and cane or hammer? I also understand why we don't want our impressionable kids watching shows like "Jackass.") The cartridge went off (Lord knows where either the cartridge case or the bullet ended up, thank God no one including myself was hurt), my Dad heard the noise, and very shortly thereafter I received negative feedback applied to my posterior in an amount commensurate with the foolishness of my action. (I have to tell you that I was tempted to start with a .30-'06 cartridge, but decided to start small and work my way up.)
Learn from my mistake. Don't shoot .22 LRs in anything except a firearm properly chambered for the cartridge.
