Apparently, the clothing with permanent shield charms are an invention of the Weasley Twins (https://harrypotter.fandom.com/wiki/Jinx-Off). So, not as widespread in the wizarding world as you assume. The article on the shield charm itself lists that most wizards can't produce a functional shield charm (https://harrypotter.fandom.com/wiki/Shield_Charm), so yeah, guess a muggle with a gun could kill most wizards. I apologize for linking to fandom.