I bought a lightweight upper for my daughters AR rifle a few years back to use as a dedicated 22 upper. We shot a few hundred rounds of 22 thru it at it worked perfectly. I then put a 5.56 bolt in and tried to shoot a few rounds of ball ammo thru it. It wouldn't cycle the next round in. I took it home, removed the gas tube which was 100% obstructed with lead. After swapping out the gas tube it we were back in business.
I ran into the guy that I bought the upper from a few weeks later and told him about the event. He told me that his daughters must have shot 10,000 rounds of 22 via a Cieners kit and that a 5.56 round has never seen the inside of that barrel.
Long story short, 22's are dirty and will over time lead up the gun.
Sasquatch has taken a picture of me.