It's called a Highpower Rifle Range, not a sniper range.
The reason for the prohibition on 338LM and 50BMG is twofold;
1. Those calibers ruin impact berms. That really can't be debated.
2. And this is where I'm going to get in trouble for even mentioning it, but those two calibers are not used by serious marksmen. The Leveridge Range is for serious marksmanship competitions, and 338LM is just not used.
People can debate me on that second point, but I think I'm on pretty firm ground.
I have head point #1 before at Reading. But not #2. The other reason I have heard is how far the round can travel. But there are other rounds that might travel just as far and are more common among noobs.