I tried out bighorn in my disc extreme yesterday. Had a few shots that didn’t fire and a couple hang fires. Using federal 209a primers, which are supposed to be the correct primers. What gives?
I read that you have to have a tight bullet fit and good compression of the load. Is it that critical that the bullet is seated perfectly? I had tight sabots and loaded until the bullet wouldn’t move anymore, then gave it a good whack for good measure.
I am going to take a stab at this... might not be right, but anyway...
You indicate you are shooting 209A's as your primer and that is OK... but totally not necessary with a Knight rifle and breech plug. Western advises using the 209A as a method of getting positive ignition all the time. With a Knight breech plug they are not necessary and in a the long run can be a detriment. I shoot Winchester W209's and really have never had a problem if I do my home work.
Are you cleaning/drilling out the flash channel of your breech plug after each range session?

The channel/opening directly under the nose of the primer leading to the flash hole can/will become clogged with a very hard carbon as time goes on. The Super primers accentuate this build up. Using a cooler primer in the BP will result in less build and more shots before cleaning is necessary. I use a simple home made tool to drill this carbon out.
I just call it a 'bit on a stick'

Simply get a small wood dowel and 5/32" bit - drill a hole in the stick then glue the bit into the stick. You will be surprised how hard the build up is. You might even have to start with a 1/8" bit to get a part of the channel clean and finish with the 5/32's bit.
Again this is only a guess on my part at this point...
Another thought.... if you are trying to use BH in the Bighorn you may always get spotty ignition. BH is recommended for closed ignition system only and the Bighorn is not a closed ignition.
Usually using a FPJ (red plastic jacket) with a 209 you can get semi-reliable ignition with a clean BP but it is still iffy. Using a bare primer ignition is really on the unsafe side because the blow back pressure of ignition can spit the hammer back and spit the primer back out of the breech.
This picture some results of using a bare prime system in a 'plunger' 'open style' breech
