Originally Posted by BarnesAddict

You can not tell it by looking at it, at least I couldn't. Of all the precision tooling I have, I'll admit I had nothing to check the size difference in the flash hole. A couple guys on the forum do measure them and maybe they'll respond.

I was shooting a tremendous amount when using Encore platform rifles each year. It NEVER failed that when my groups started to open up (consistently), the replacement of the BP ALWAYS brought the groups back together. Always. Note: Sometimes you just have a bad day. At times it may take only 100 rounds through one for it to start opening up groups, other times I might get away with 200 or 250 rounds. A local sporting goods shop kept a steady supply of BP's in stock, knowing I'd be right back.

Now I'm in no way saying this is your issue. Its just one possible contributing factor. I've seen guys start to have problems and sometimes it was the BP, other times it was his methods changed. Many times its mount, rings or a scope problem. I always suggest checking the mount, rings and scope. Muzzleloader can play heck on scopes and I certainly know about that myself. I learned that making sure that the scope tracked properly was very beneficial and saved me a lot of time and money.
Start with the basics, ensuring that you're using good propellant, that your mount and rings are torqued TO SPECIFICATIONS, with a torque wrench. (Caldwell makes a reasonable priced unit) Then I'd check the scope and make sure it tracks properly and that something's not wrong with the scope. Check the tracking by knowing exactly where your scope turrets are now. Dial up the elevation a full turn, then back to zero. Then dial it down a turn and back to zero. Do the same with right/left. If the scope is tracking properly and in good condition, you'll retain the same zero that you have prior to making the adjustments and your group, regardless of size, should go right back into place.