> Vacuum kills pretty quick.

No, it doesn't. The overwhelming majority of instant death scenarios would be collapsing of the lungs. If not, you have consciousness for about 15 seconds since your bloodstream still has oxygen in it which we have evidence of, not to mention rough calculations of oxygen saturation in the blood.

If you still have your lungs though, it's estimated that you can survive in space for about 2 minutes without permanent damage (ie significant loss in function).

Though, with regards to brain damage - generally you can survive 5-10 minutes deprived of oxygen without significant loss of brain function. The upper limit of avoiding brain death from oxygen deprivation is around 20 minutes.

However the dangerous bit here is primarily that you'd be on a spacewalk, meaning it would be near impossible to retrieve you in time. Not only does putting on a spacesuit take a significant amount of time, but they operate at a far lower atmospheric pressure than the space station, so they'd be fighting severe decompression sickness at the same time. It's hard to say though, because I don't know if they have any sort of procedure for that type of thing.

It's definitively not the only double-barrel cannon in the world, because even if you argue autocannon that exist most notably in naval ships are somehow a different category of cannon and don't count, a modernized replica of this exact cannon was built on the reality show American Guns.