Parker Brothers was never officially involved…
U.S. Army Prototype Anti-Armor Hand Grenade from 1973 – A Shaped Charge Packed in a Hollowed-Out NERF Football:
“Since a regulation size football weighs 14 ounces, it was considered feasible to make a shaped charge grenade within this weight limitation,” according to the official test report. “In addition, most U.S. troops are familiar with throwing footballs.”
Footballs, however, are not solid inside, making the prototype grenade unstable in flight. The project was cancelled and Parker Brothers (makers of the NERF footballs since 1969) was never officially involved.