Let’s see…in 2021, depending on who you ask, between 103 people (FBI) and 700 people (Gun Violence Archive) died in mass shootings. This is as compared to 48,830 deaths from firearms total. So ~1.5%.
As for handguns: “In 2020, the most recent year for which the FBI has published data, handguns were involved in 59% of the 13,620 U.S. gun murders and non-negligent manslaughters for which data is available. Rifles – the category that includes guns sometimes referred to as “assault weapons” – were involved in 3% of firearm murders. Shotguns were involved in 1%.”
So above poster was a bit low for purely murders. However, mix in the fact that about 54% of gun deaths are suicides (overwhelming handguns), and the total percentage of gun deaths attributable to handguns is about 80%.