As far as I am concerned, even befor WW1 a number of automatic and semiautomatic weapons existed. Yet the standart rifle of most armys were bolt action rifles like the Lee Enfield. Especially in WW1 I think it would be advantageous to use a semi or full automatic weaponin trench warfare . So why were bolt action rifles the standart issue rifle in the world wars?
There definitely were fully and semi-automatic weapons before either war broke out, but especially before the First World War, there were still a lot of problems to solve before they could become viable. More can always be said on the matter, of course, so if anyone else would like to address the matter of firearms in the first half of the 1900s, please don't let this post stop you! For the meantime, OP, here are some previous threads that look at rifles: