When your assembling many a day its going to be a mallet, hows even a gloved hand going to feel after 20-30 palm smacks!Maybe not, who knows. Closing up with a mallet is apples and oranges, however. A couple of my new stocks are best closed with a mallet, though I use a rubber one.
Would not surprise me if they had a jig that pressed on the gas cylinder then a hydraulic screw driver driving the screw home