They can be effective, it just depends on where the round lands or if the driver gets the hint (that’s another phenomenon you’d be amazed by).
The “working the action for some reason” thing you refer to is the M2’s headspace and timing being out of whack. Improper headspace and timing can very easily screw up an M2 and turn it into a bolt action rifle. It’s fairly simple to correct, but it takes a precision gauge (usually on the a gunner’s keychain or something similar) and a few minutes of time without enemy contact.
In 1978 I had to investigate an accident that occurred aboard a minesweeper in the mid-Atlantic. A Gunner's Mate had set up a .50 on a pedestal mount for some gunnery practice but he was in no way trained to do so. The first time he tried to chamber a round. it failed to fire. He tried again and the round fired about 1 inch out of battery. The brass fragments from the cartridge case riddled his groin and upper legs and he was bleeding out from dozens of wounds.
A Soviet AGI intelligence (spy) ship was in the vicinity and heard the emergency call and transferred the wounded Gunner's Mate to their hospital spaces and saved his life.
One of those few good moments in the Cold War.