According to Wikipedia, the Austrian Social Democrats and their allies had
- over 2,500 rifles,
- 250 revolvers,
- 1,500 hand grenades, and
- 10,000 rounds of ammunition
in February 1934.
I suppose that soldiers returning from World War I may have kept their weapons. But I assume there was some sort of oversight (a soldier coming from the front couldn't just say "I'll keep the rifle for me").
Corruption is another possibility: I can imagine some arsenal guard selling rifles and ammunition to support his family during the famine of 1918. Also, these paramilitary organisations were parts of parties at the power. It's thinkable that some politician would order that arsenal employee to give weapons and ammo to whomever the politician wants.