Sound like air rifles to me.
If memory serves, it's an offence for his pellets to leave his land boundry, so if any go astray into your land, call the police and report it.
They do not like people having lethal weapons such as air rifles so normally respond well to complaints, often confiscating the items to "check" their power level. Can be a right pain to get them back in one piece
However, if he keeps it to his property, its legal (assuming their power is under 12ft/lbs or if over, they are both certified)