What, you're looking at a 21st century prop gun, taking your knowledge of 20th/21st century fire-arms and trying to draw a conclusion. But it's sort-of foolish to do it because Vera isn't a 20th/21st century gun! The prop
might be built using prop-guns of 20th century weapons but Vera, the gun in the Firefly universe exsists hundreds of years in the future. We've no idea how the guns of the future look, work or how they can work/not work in a vacuum.
You're looking at a prop of a 20th century gun and trying to say how a 24/25th century gun should work. I could do the same thing with Chekov's brain-fixer. "Hey! What, did the medical technology makers stumble across some old moel-kits of a Klingon battlecruiser?! Didn't they find it odd that these things exsisted centuries before Klingons were known about?!"
"Hey! Does Mr. Fusion work by grinding up the trash into a really fine powder and then make the energy by filtering hot water through it?"
It makes as much sense. We don't know how Vera works or how any of the guns in Firefly work. They're 25th century
weapons being represented by 21st century props.
I doubt the "Mule" is "really" an ATV from 20th century Earth either.