I use musket caps in my remington. I have shot Pyrodex and Cleanshot pellets this way and had no problems with it. I would think 777 would be about the same, but they do suggest 209's instead. If you were to get a missfire at the most inopertune time I wouldn't try and blame them.

Mark is right, loose powder is better anyway. Pellets are just more convenient is all. Of course they are more expensive as well, and you can't tailor your load for accuracy. And with 777 the powder is faster when loose. Two 50 grn pellets of 777 will be the equal of 100 grns of pyrodex pellets. However when shot loose 777 is hotter for the same volume compared to pyrodex. And cleanshot or pioneer as it is called now is even weaker than pyrodex.

I use loose 777 with musket caps and it works great, good luck with whatever you try.

Paul