“Since a regulation-size football weighs 14 ounces, it was considered feasible to make a shaped charge grenade within this weight limitation. In addition, most US troops are familiar with throwing footballs,” according to the Army’s test report for the weapon.
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Footballs fly through the air because there is an even distribution of weight surrounding the hollow inside of the ball. But 14 ounces of explosives tended to make the trajectory of the Nerf grenade “unpredictable,” according to the test report.
It would but harder to make a shaped directional armor penetrating charge that way.