I reuse the plastic discs multiple times when I am at the range. They do loosen up a bit with repeated use. That can be a good thing because you don't need teh wrench to remove them then. But it can also be a bad thing when humting because you don't want them to slip off or to allow any mositure to get inside and ruin the priming or (worst case) the powder charge.
So - what I do is use NEW disks to hunt with but save the old ones for use while practicing at the range and those I have used 10 or more times.
When I heard that knight was going out of business I bought a new box and began to be more frugal with their use. So now I re-use them a lot more than when I thought there was an endless supply of the disks readily available at the local sportshop!
I would second the motion to what Mike said. I don't have one of these fine rifles, but I have a good friend who does. It's all he uses, and I know that he reuses the discs all the time. He hunts with his muzzle loader in three different states, and so he probably shoots 15 to 20 times a year (he's a good hunter and gets several deer every year). I can't say if he's ever replaced any of these, but I know that reloading them and reusing them is just part of the game for that kind of rifle. They seem to work very well for him.
I must admit that I have missed a big game animal with a rifle. And not only was it a bull elk, it was the largest bull that I have had in my sights with a tag in my hand. But, as with most "missed shot" stories, I have an excuse. Moisture doesn't really ever bode well for hunting equipment. When moisture (either rain or snow) gets into a firearms barrel, nothing good takes place. A couple rain drops down there can change the point of impact drastically and a barrel jammed with snow after a...