This is why I say 'metre' for the measure and 'meter' for the measurement device

In the same way a US ton and a metric ton is like 10% different, a 556 bullet is actually 5.7 mm across.

I would make a bet that more mass shootings are done with 9mm. Depending on which shootings they consider 'mass' I see estimates from 60-80% for handgun usage. I'm sure the cheap .22 is a large number, but 9mm is probably right up there. There is a large bias in reporting the school shootings and shootings involving rifles by the media. They almost ignore the others.