"Always turn your No Pew on, boys. There's nothing worse in war than Friendly Pew Pew."
War is no joke. Soldiers, however, are hilarious. Think about how many terrific words we've gotten out of the gallows humor shared by grunts. Like the cute little word for a screw-up, "snafu." (Bless you.) It's something your grandma might say, but it stands for Situation Normal: All F-ed Up, which is still kinder than it's more severe cousin, fubar (F-ed Up Beyond All Repair). The names they give weapons or vehicles can be even better. The list is too long to go into, but our personal favorite was a nickname for one of the very early helicopters: The Frustrated Palm Tree. So, our soldier friend here comes from a long line of GIs who dealt with the violence of war and the mind-numbing absurdity of the military (See: "chickenshit" - all the little tiny rules they make you follow that would be bullshit if they were bigger) with good old-fashioned American sarcasm.
Note: We're comedians and we don't know about guns (except that it's a Bushmaster AR-15 from their discontinued plant in Maine - eat that, gun people). We can't tell, but this doesn't look like it has an actual "Pew pew pew" setting, just "No Pew" and "Pew." That would probably make this a civilian model. However, we're going to ignore that and write this nice piece about how funny the military is because we don't know enough about guns to say it's not military.
Sources: Redditor Myuagi