Not something I would try. But, with a .50 caliber bullet fired through a ~.700" bore there would be much less chamber pressure, absent the resistance of bore and rifling. The bullet was launched forward, but at significantly lower velocity and energy than would result from proper use in a Browning M2. Zero stability in flight, likely to tumble to the ground before travelling very far.
Well that was entertaining, and interesting, I must admit. It’s easy to flame this guy, but with the use of string and a rest, he didn’t really do anything that was unsafe, except you know, the part where he loads a 50 bmg cartridge into a 12 gauge pump.
Well, now we know! LOL
Why not? Everybody has to be somewhere doing something. I'll bet the farm against a hole in a doughnut that everybody has done something that somebody thought was stupid. Larry