It happened twice before and your dad still wasn't smart enough to figure out that if he wants to breed her he needs to keep her away from all other dogs (and obviously not out in your yard) when she's in heat?

Having an airsoft gun near the door as a way to stop her getting pregnant is the dumbest, most braindead solution you could possibly have dreamed up. Not to mention it being cruel and having the possibility to hurt your own dog.