That was the reason given to us by the BoD. The backstop is otherwise rated for the caliber. In any case, no good reason to shoot .50BMG in an indoor 50yd range, so the ban didn't cause an outcry
Oops, sorry, I misread your comment, I thought you were referring to concussive blast of mils not 50. My mistake. (I have a 50....)
As for steel core, is there rubber in the backstop? If there is, that might be the reason for no steel core.