I asked about these in the Muzzleloader Forum and did not get positive responses.
I too am looking for a new MZ bullet. First, I freely admit I am stuck in the past. I grew up MZ hunting with Hawkin side-hammers that threw huge chunks of lead in front of real black powder. I've moved up to an inline MZ, but still like to throw huge chunks of lead. I'm still using real black powder but plan on switching to 777 for easier cleaning. For years I shot the 295 Powerbelts due to their full 50-caliber size, ease of loading, and incredible accuracy. However, the 295 Powerbelts go to pieces on impact with a deer. Five broadside hits on does at close range, none produced an exit wound, none produced a blood trail, and all five fragmented into almost powder inside the deer. Everyone in my camp switched up to the 348 Powerbelts while I moved up to the 405 Powerbelts. Problem solved. Full pass-throughs, good blood trails. And then Powerbelt stopped making them (as of last year). They're only going to be producing their crap 295 and 245 bullets. Why, I don't know.
Again, I'm stuck in the past, and absolutely refuse to shoot a pistol bullet in a sabot out of my MZ. My MZ is 50 caliber and I want to shoot a 50-caliber bullet that punches completely through a deer. Many of the specialize saboted MZ bullets being produced will not do that at "normal" MZ loads. I also refuse to turn my MZ into a modern rifle by using ultra-hot loads are smokeless powder. If that's the way we are going, let's do away with MZ season and just go right to modern centerfires.
I've been searching for a new MZ bullet for several years, as the 348 and 405 Powerbelts became more and more scarce. I've not found one yet. I could always go back to pounding huge chunks of lead down the barrel with a hammer (literally), but I'm hoping someone comes up with a bullet similar to the Powerbelt - a bullet that is full-sized but only engages the barrel's rifling at firing.