They'd have been mowed down and killed. WTF do you think would have happened?
Normal Jewish citizens with guns would have had NO CHANCE against the SS, Hitler's regime, the German army, or any of those. For every German that was killed, a new ghetto and hundreds of Jews would have been burned to the ground in payback.
And FYI, a sizable number of Jews did have guns, despite there being laws against it after 1938. Didn't seem to do a damn bit of good.