Borderdogs,
Something to look at would be is to see if the sides of the muzzle cap that are along the side flats of the barrel are too tight. If they don’t fit right, it may be pushing the cap down.
If the barrel was inlet well, and sat correctly before you put the cap on, then something is wrong with the cap.
Or, is the cap somehow twisted? In your picture, our right side (left side of gun) it looks like the side flat sits correctly, but the other side does not. Is it either too tight width wise, or twisted somehow?
Is the right side of the barrel pushing the cap down?
A little filing might solve the problem. If you were going to trash the cap anyway, then filing won’t lose you anything.
Norm.