Considering the present cost to bring a second item as checked baggage these days - or dare I say the first item - shipping becomes a convenient alternative. You cannot check rounds onto the airplane, but you can ship them by ground freight inside your musket box. Since I'm shipping, I usually throw a few other items into the box to minimize what I take on the plane.
You don't need an address to ship your box. UPS can hold it for five business days if you advise them that you are doing such. Ship it by UPS ground to one of their customer service centers about a week and a half before the event, and you're golden. You can return to the same or a different customer service center after the event and ship it back. Just remember to throw some extra tape into the originally shipped box so you have tape to reseal the box for the return trip home.
I've been shipping muskets and rounds like this for years. You don't need a box from cabella to ship. I use two layers of old fashioned cardboard and lots of tape. I wrap my musket inside my groundcloth, blanket and canvas then shove a bunch of newpaper into areas which need reinforcement.
- Silas Tackitt
"I take [this hobby] seriously just as any golfer, hunter, or fisherman would take their hobby seriously. You whine that "hard kewls" are what is wrong with this hobby, but I digress. In my opinion, the major defects in our little world consist primarily of the clowns and jackasses who don't take it seriously." - Mike Phineas, 2 August 2013.