HMGs aren't AV, I believe you meant Autocannon's, but thats beside the point.
Almost all AV weapons are carried by squad specialists, which means there is a very limited number on the field, given that many vehicles have tactical sense all squads with specialists armed with AV weapons become your priority, let alone snipers who can shift target priority to the specialist model himself not just his squad. Because of the limits of the number of specialists really no more than about 25-28% of your army can be specialists, and should those models be eliminated victory for the side with the vehicle is practically guarenteed. Although this would be the one rule that could prompt me to go up against a Huricane Walker with my Holy Carnager Polearm wielding brotherbound (Antitank spear of doom!).