They'll more than likely be a flop. I believe they are only available in 350 grains and thats it. They fit all MLer bores, right off is a lie.
And of course you have already tested this theory of yours. I guess you don't understand the principle of the design. The back is shaped like a cone, they probably start off in the .505"+ arena, once inserted they will swadge down to bore size whether that be .499" or .504". The majority of the bullet is probably in the .500" diameter range, and through obturation will belly out into the grooves.
Lane makes a good point about the seal, that may prove to be a challenge. Some lube in the base or a fiber gasket wad would take care of that worry, but would include an extra step in loading. Those fiber gasket wads will seal anything, but may hinder the obturating properties of the design.
I do know one thing, I have tested several muzzleloader bullets at various ranges, firing into different types of media, the PowerBelts are the King of the WORST. You never know what one is going to do. Why limit yourself to that?
If you look at he new Hornady FPB, they are touted to work at any speed between 800 fps and 2000 fps. This means any load you want to use up to 150 grain charges. I hope they live up to their word, because I know I will surely be putting them through the wringer. I will even do the side by side comparison with a PowerBelt, I already know one bullet isn't up to the task.
http://www.hornady.com/media/2008catalog/Page_0043.pdf