My ex-step-father told a story of being in the Ohio National Guard in 1960 and manning a recoiless rifle mounted in the back of a Jeep? They meant to fire a spotting round and fired the main gun instead. The Jeep was not set into position and rolled with the crew. I don't know if it was true but it was a good story.Not my gun, I found this for sale on Gunbroker. Thought it would raise some interest and comments. I have never shot a 20mm or 30mm but was a gunner on a 106mm recoilless rifle back in my Seabee days. The main thing you had to remember was to pull the firing knob to shoot the spotting rifle and push the knob to fire the 106. If you called out that you were firing the spotting rifle and fired the 106 by mistake, people around your position tended to get a bit testy!
Ya my understanding was the gun was originally the Boys rifle in .55 caliber which had to go under NFA rules because it was over 50 caliber. So guys converted them to 50 caliber and no NFA rules.I don't think they made them in 20mm. An original was .55 cal and often converted to .50 BMG. Hard to imagine converting one to 20mm