Somebody makes one now... A few do I think. Doesn't Sico? Or is that just for .458? There are low pressure .50 cans though, I've seen 'em. Hell, there's even one for an M2HB believe it or not and that's a blowback operated weapon. It was glowing after 100rds. but it stayed intact. I wouldn't be surprised if Liberty made a lightweight monocore model. I have a Beowulf and I contemplated it but changed my mind. I just don't use it that much. A BMG can weighs about 4lbs (and so far, that's been regardless who makes it and regardless of the material too best I can tell). Not sure how well it'd do with subs but I know people load BMG subs (I think you have to use a case filler?). I know what you're talking about, .510 Whisper, right? I don't even know anybody with one.... But I'd sure as shit like one since I already load BMG. And I'm guessing you need a BMG press just due to the size to load for it?
If TBAC had made a BMG can a few years back I'd have probably gone with them, but I can't complain about my Elite can. It's a tank if nothing else. When they do come out with a BMG can, I'm sure it'll take center stage (and be less than 4lbs!).