So I have a knight bighorn that has a musket cap nipple. Can I just swap nipples or do I need a different breach plug?
Now remember this is just my own opinion.... I would totally agree with your swap to the #11 cap - just remember that you are going to want #11
Mag caps or even better the German RWS Dynamint Nobel #1075 caps.
It is a common belief that the Musket cap creates greater heat than does the #11 - well which is true but not in the case pf the two caps mentioned above. They are just as hot or in the case of the German cap slightly hotter than Musket Caps.
Another thing you need to be careful of is the in the recent time most Musket caps have gone back to 're-enactment' strength which has reduced there heat.
With the Bighorn it is an easy swap to #11 - and better yet if you get the right nipple and apply the cap with a capper you can virtually make the breech water proof. The caps fit so tight the convolutions on the cap are stretched tight on the nipple, making it virtually water tight. But, there then is problem - the only way you are going to get the cap off is to shoot it off or peel it off with the edge of a knife blade.
This pic shows the convolutions of the #11 cap
This is the nipple I would suggest
This one of the Track of the Wolf #11 nipples installed in a Knight
Oddly enough the capper I would suggest is sold under the 'Traditions' name.
https://www.sportsmansguide.com/product/index/traditions-performance-firearms-muzzleloader-field-capper-10-11-caps?a=434694&utm_source=Bing&utm_medium=cpc&utm_campaign=Bing+DSA&pm2d=SEM-SPG-DSA-BING&mkwid=BI2BBJsO&pcrid=78271493776485&mtype=bb&devicetype=c
