The guns used by the main characters actually fired blanks.
The Stormtrooper rifles were modified Sterlings with 5 or 10 round magazines.
The longer guns were MG34's.
They used blanks so the actors would have some "feedback" when they fired the "lasers".
It also provided a queue to sync the optics in post production.
Before Lucas "fixed" the original movies you could see ejecting brass when they fired and in a couple of scenes you could see brass on the floor.
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