Unrealistic implementation will cause unrealistic behaviour. If a realistic feature is included in a truly realistic way, then it will not.
There is a reason why machinegunners would not have used their MGs like SMGs or automatic rifles. If you can model those reasons in game, then players will have the same incentives to use their weapons properly.
Since MGs were heavy and more unwieldy than SMGs, giving them a significantly longer sight-in time and slower player movement would be effective means of preventing MGs from being used as offensive close quarters weapons unless the player was really desperate. You could also prevent players from walking while sighted (unlike other small arms).