I have absolutely no experience with any machine gun tho I once had a Beretta .22 short go full auto by accident. It fired three shots in a burst so short that I had to count the ones left in the mag to know how many actually fired.
I do remember a long video from I have no idea where. It was of the lead tank in the advance on Baghdad. A guy was manning a .50 and was half way out of the turret.
As they moved along, he blasted several vehicles and stopped them cold. I can still recall him firing just a few rounds at a pickup truck. After stopping he would for some reason have to work the action again to prepare for the next target (victim).
The tank never used any other weapon but that .50 just kept knocking everything out.
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.