"A current military helmet, such as the Advanced Combat Helmet (ACH), can withstand an estimated energy of 3.2 Mega Joules before breaking, which is roughly equivalent to the energy of about 0.7 Kg of TNT in direct contact. However, the actual performance can vary based on the distance from an explosion and the angle of impact."
lot of if's with that. even if the suspension is new and correctly set up it could still snap your neck or blow out discs. depending on weight of hail stone , position of your head, velocity of the hailstone, cause you know its falling at terminal velocity, minimum it would probably knock you out cold.but a my choice for a helm would have to be one of these. jus cause its cool.
with the articulated plating over the neck area....extending down....