Revolver 'technology' is MUCH older than automatics for guns. Pre-Civil War. So, from a hex pov, more reliable. (This does conjure up the image of a cunning hitman using a civil-war replica Gatling Gun with .22 ammo to gun down wizards...but I digress.)
Also, revolvers are easier to maintain. Going with my 'micro-threshold' idea, it takes less time to keep them functioning around a wizard. (They also don't leave shell casings around for pesky police forensics.)
re hexxing armed opponents; it's rare that we see ONE gunman sent after Harry, or anyone else. So, while you're hexxing a gun, the other people are shooting at you...or you could run, get behind cover, throw up a shield, etc. and protect yourself from ALL the shooters.
'micro-thresholds' also explain why Michael has ANY spare time, given that Molly is living at home...