The shooter quickly notices the absence of recoil and noise. An extended burst, usually 30 rounds, can be placed on the desired target with ease. An ammunition cost of less than 60 cents per magazine may allow you to encourage new shooters to machine gun ownership without putting a dent in your budget. In order to maximize the shooting enjoyment of the M16 with a 22LR conversion, you need a dedicated upper receiver for it. You also need a way to make the conversion unit reliable.
Unreliable guns are frustrating and quickly become unused guns.
A dedicated upper receiver can be assembled using many of the parts you already have around the shop. When firing 22 LR ammo through a 223 caliber upper, lead builds up at the front of the 223 chamber, in the bore and in the gas take off vent . This lead buildup can result in dangerously high pressures if you fire a 223 without thoroughly cleaning it. A separate upper for the 22 LR conversion allows you to quickly change between calibers without having to worry. The dedicated upper also allows you to have a configuration not possible with a 223 upper.