The slug from a .22 bullet discharging without being in a gun is not going to develop any velocity. As soon as the slug separates from the casing all gas pressure is lost. It probably wouldn’t travel 20 feet. So this guys story about it knocking him over, etc. is pure hooey.
This thought crossed my mind as well, but I have no experience with what level of greatly reduced force (with no barrel to allow pressure to build) a .22lr would generate, and I was wondering about larger cartridges. Guess it would depend on the seating depth and type of powder used.