This page will describe the differences better than I can.
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Also the M4 or M-16 IS basically a .22 The cartridge it is designed to shoot is much larger but the projectile is 22 caliber. An M4 can fire 22LR with a after market bolt that replaces the entire bolt carrier group. Of course you also need a special magazine that will fit the mag well and load the 22LR rounds. No other modifications are needed.