It was even test-fired once and supposedly hit a test satellite.
Apparently the other military Salyut, Number 5, did not carry
such a weapon. At least that is what they tell us. I wonder
if the two failed Salyuts before Number 3 had weapons as well?

http://www.fourmilab.ch/documents/spaceguns/

http://en.wikipedia.org/wiki/Salyut_3

http://www.astronautix.com/articles/thistems.htm

Did the Soviets really think the US was going to try to board their
space station and take it over? Skylab, which was much more
sophisticated and roomier, was still orbiting Earth at the time if
the astronauts needed a place to stay.

I am being a bit facetious here. Apparently the Soviets were not
worried so much about a manned American boarding party as they
were a robot satellite snooping around their station or even trying
to attack it.

And hey, if an alien invasion ship tried to get the station, at least
they could do a bit more than throw a space wrench at them.