I kinda believe that is a .50BMG parent case necked down a little more than a .30 cal ... more like a .22 cal. Either way with that case capacity (a .50BMG has over 5 ones the case capacity of a .308Win) I'd go ...
.30 Incinerator or .22 Incinerator
... because while it will burn the barrel out like nothing else the bullet will be incinerated a very short distance once it leaves the barrel at the velocity that thing would be traveling. Now if you could find some turned special brass alloy VLD extra long you could survive the velocities that thing would reach ... if you worked on a proper load that thing could break speed limits but the barrel, especially the throat, would be burned out in super low round counts.