With ammunition, the period goes before the number, e.g. .22 caliber.

Really hard to say for sure if the effect you want would happen, but I'm willing to accept that it could result in a painful wound but not death. I remember reading a news story about an irate driver shooting at a guy with a .22 handgun, and the guy just getting angry and giving the shooter a whupping. So to hear of someone trying to kill themselves with a .22 shooting blanks sounds like a half-hearted suicide attempt at best, and at worst, a stupid comedy skit. If this is actually serious, if they're trying to kill themselves, your character really has to up their game.