5.56 NATO I can understand but the .58 cal I can not.
Currently there is a 6.8 SPC variant and the 6.8 SPC has ~+30% more Kinetic Energy than the 5.56.
Though I can understand the temptation for a .50 cal variant a 6.8 variant makes more sense. The Steyr AUG is not known for sniper quality accuracy (it does not have a free-floating barrel).
Now if you used a model that does currently use .50 cal ammo for instance the Barrett XM500
then I will not bother to offer objections. After seeing your Stery AUG model I would like to see you take on the Barrett XM500.