I'm 100% certain that in a Guinness Book of World Records a good few years back (2004 - 2008 possibly) the US Navy held the record for the fastest projectile - it was either 225,000 mph or 425,000 mph but didn't elaborate on what was being fired or from what. Be interested to know if anyone remembers this.
The US navy have their experimental railgun, which shoots at around mach 8 (3700 mph) but I think what you *might* be thinking of is this; https://www.nytimes.com/1994/03/22/science/fastest-gun-on-earth-goals-go-beyond-planet.html
Essentially, they were launching microscopic particles of dust to measure the effects of micro-meteors, or other bits of microscopic space-dust, hitting space station parts, to measure the damage, although it doesn't say whether the tests were conducted inside a vacuum or not, interesting reading though.