If you look in countries where guns are banned but are with comparable size to USA you will see mass murders by other methods, even if they are more infrequent than in USA (for example mass stabbings, bombings, arson, or more than once in Brazillian schools: mass murder using axes, and at one time, a crossbow)

Then there are mass murders using vehicles, like it happened in Europe several times in the past 15 years, or mass destruction with vehicle, like the "Killdozer" in USA itself.