The 22 isn't optimal at all but it WILL do the job in one shot if its really needed I have a friend serving life in prison that will attest to that. He used his 22 when he was in an unstable state of mind "drunk and higher than a kite" it took one shot from 10 feet guy dropped like a sack of bricks. He plead guilty and got life because of a "violent" history for an assault that happened 10 years earlier. Now point being when I personally asked him what the hell he was thinking he said dude it was a ****in 22 I didn't expect it to kill the guy. Just because its small doesn't mean it doesn't work. Now I'm not saying my friend is a nice guy or good person by any means all I'm saying is a 22 will kill an attacker given the shot is placed correctly and I would carry one anywhere anyday rather than carry nothing.
I prefer my .45 but sometimes carry my pf9 on hot days. So I'm not saying the 22 is th best round but it is sufficient if its the only option for whatever reason.