This is a smart man. You can also save money when you train with it. If you shoot someone with any gun several times they will fall. And think, a .22 is a slower M4, same bullet, less powder.
As the others mentioned it is not the same bullet. But don't feel bad because bullet and cartridge sizes can be really confusing. When I first started shooting I was so concerned I was going to buy the wrong caliber for my firearms. Nobody here knows it all--that's for sure. Take a look at these pages, which describe each. You're right that the .223 has more powder behind it and that makes the bullet travel about three times as fast.
Here's the .223
http://en.wikipedia.org/wiki/.223_Remington
and the .22 LR
http://en.wikipedia.org/wiki/.22_Long_Rifle