I do not claim any special knowledge about anything but here is my personal experience using .22 ammo in my legal Federal registered, tax paid, etc H&R M-16A1.

I purchased one of the first .22 LR conversion and boy was it fun! In about 15 seconds zzzzz-p, 25 rounds disappeared downrange!

There are a lotof conversion devices out there today! Are they a good purchase? Hell no!

My M-16A1 upper was a 1-9 twist. The .22LR used a 1-16 twist. One mag of .22LR down the barrel and you were shooting through a smoothie barrel! Accuracy was gone!

Personally, I have a idea that a few rounds of 5.56/.224 would open the lands and groves...