well, the austrians had a sub-mg, so if the germans and austrians move the development a little bit ahead, they have em... if the germans have em, the french will capture it. Learning how effective this weapon is, they will copy and improve it...
if the germans have them in larger numbers BEFORE 1914 it could be a war-winner, if they develop it in 1915, massproducing it in 1916 it still could change the war, but also the french could react fast....
automatic weapons will nihilate any mass attacks... if the defender as not only a lot mgs but also a lot sub-mg, the attacker can do nothing.
so either the tanks or new tactics will be developted much faster...
but not to early... or you change everything (germany wins fast, so france has not time to build em, germany forbid the french to have em, etc... you know the game) also it is moving to asb... such modern weapons need a basement, the trench war was the push... so you can push it for 2 years, say in 1916 the germans use em, they loose some in verdun, but only after the big british defeat at the somme, with losses around 100.000losses at the first day (cause the germans have gave the troops here such guns and they massacre even more tommies as the historically 57.000,) all combatants try to improve and build them...