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In high profile mass shootings. I don’t know if there is data on it but I’d assume most mass shootings are committed with handguns.
https://www.washingtonpost.com/nation/interactive/2023/ar-15-force-mass-shootings/
32% increase in ar use of ar rifles since 2010 https://www.thetrace.org/2023/07/mass-shooting-type-of-gun-used-data/
The first articles overview says it looks at 11 mass shootings and the 2nd link mentions that the data is from the narrowest subset of mass shootings. Without a strict definition you can have the numbers point to what you want.