I'm afraid I have absolutely zero knowledge about weapons so I can't help you with your question. But I'm sending greetings from Germany and a welcome aboard from The Traveler's Companion and Other Notable Biographies forum!
I should have thought of this earlier. A rifle with a 33 in. barrel, deep 7 groove rifling that looks 7-sided, and .54 caliber sounds like it may be a M1841 Mississippi Rifle. But the Mississippi was originally percussion and not a pill lock. We are looking forward to photos.