Not sure if this will help or not... but passing this on. I think I see what you are indicating in your picture. The corrosion that I am looking at is really nil and appears to be rust dots.
When I clean the BP port I use this tool.
It is a 16 gauge shotgun fiber brush on a piece of ram...
It truly may be a problem with the way the hammer assembly might be installed and/or the length of the breech plug and nipple setup.
First thing I would suggest is to check the secondary safety on the rear of the bolt - make sure that it is turned all the way out. Next open and close the bolt...
As GM54 has already suggested...
A 16 gauge fiber brush installed on the end of cleaning rod extension and Birchwood Casey 2-in-1 bore scrubber. Turn the bore brush CCW.
This really works well for me.
Usually but not always the Vent Liner itself does not plug. The Flash Channel on the other hand does! It is not the normal 1/8" channel it is 5/32" actually it is slightly larger. Without looking I am pretty sure it is a #21 bit.
You are correct Western did recommend Mags with most rifles to get ignition. But the Knight BP's were built with BH in mind. That is why the Flash Channel is a greater diameter than most.
Which BP do you have a Knight Factory or a Lehigh aftermarket. Guess I should have asked if you can...
Yes, the 45 creates more pressure more quickly especially with a projectile heavier than 260 grains. The heavier the projectile the more efficient BH is. The smaller diameter 45 barrel reduces the expansion area in the bore for the pressure and the easiest way out is back through the breech...
Which rifle? (assuming a Knight) and which weight projectile?
In todays world with primers really hard to come by - you probably have to use what you got! But Mag primers in a Knight system is asking for problems. They are short so they do not seal themselves in the primer pocket. They create...
the rifle was designed to use all primers, but European primers are a little larger in diameter. This can be an issue. To test remove the BP clean the primer pocket. Push the Cheddite down tight into the primer pocket with your thumb. Then pickup the BP and turn it upside down - the primer...
If you might be interested in getting one of these follow this link
https://www.modernmuzzleloader.com/threads/knight-original-ss-super-disc-45-fast-twist.39641/
This is what I use. I have used other but I always return to this one. Other will have other suggestions...
Thompson Center Brass Volume measure.
The BIG thing is to develop a method of filling and cutting the powder with the sliding top. Once developed repeat it every time.
I would add one thing remember that you can shoot full bore lead conicals as fast as other bullets. The will cause leading in the bore.
Another thought you might want to use a wad of some sort under the bullet between the powder. The heat that BH creats will melt the bottom of the bullet...
Again from way out here in the west it really does appear that you might be crushing them much more than necessary. A couple of them really appear to be flattened and have a fairly deep pin impression.
If you has some other primers, the Winchester is the longest on the market and does a fairly...
To me your pictures indicate that you do have a seal at the base of the primer and the seat in the breech plug.
So now I am still concerned with the flash channel! I really wish you has a Lehigh after market BP so you could remove the vent liner and visually inspect the channel... again...