Brunoshooters shows h1000 in stock in 1lb canisters. There have been a lot of complaints on here about their shipping times and communication, but they say on the main page of their website they are 3800 orders behind in processing. I ordered some 300 gr OTM's, and had them in about two weeks--considering how far behind they are, I thought that was great! Anyway, Shawn Carlock has apparently been experimenting with VV N570.....
LRKM - 3000 yard setup w / supressor
You might see if you can get your hands on some of that.
With that long of a barrel, I can't see where you would have a problem with using the 50bmg powder.