Try every swear word. Not one of them fits.

The fail is NOT the placement of the billboard next to a church. The fail is what the billboard says. No swear word makes sense in this context. "This gun's not gonna sh* itself!" Good job. Potty-trained gun. "This gun's not gonna pss itself!" (see above) "This gun's not gonna F itself!" really, Duke? Really? "This gun's not gonna vagina itself!" (ok, this one miiiight work) "This gun's not gonna cock itself!" (it's not a swear word to cock a gun) "This gun's not gonna felate itself!" C