Some of those American massacres go back to the 14th century. It's quite possible that more have occurred in the USA than the UK, but that's hardly a fair comparison.
I then provided a link to a list where you could pick out the dozens of cases of recent massacre in the United States (among a list of around ~125 across history). I haven't compiled an exhaustive list myself, but just from glancing at it you will be left in no doubt as to the scale of the problem in the US and that the UK is not comparable.
Honestly I'm sure you could name off the top of your head over a dozen during your lifetime that occurred in the United States.
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