FWIW, I've owned several patent breech T/C caplock/sidehammer rifles over the last 40 years, and my current .54 Renegade is the 2nd .54 Renegade I've owned.
The T/C threaded breech plugs are investment castings, as are the patent breech/tangs, and fitment can be problematic if either the inside of the patent breech recess or the outside of the breechblock plug hook has a crud buildup or a casting seam standing proud.
Even though your breechplug appears to be tight (and therfore, safe), the gap isn't either normal or desireable, and the hook/recess should be examined under direct light and/or magnification to resolve any fitment question there.
While not a safety issue, it can certainly be a cosmetic issue.
The .54 issue barrel can successfully use patched round balls, unpatched T/C Maxi-Balls or Maxi-Hunters(HP's), or even saboted .45 JSP/JHP pistol bullets.
BP substitute pellets can be hard to ignite, making loose powder a better choice in a sidehammer - AND the charge can be more easily adjusted for better accuracy (less) or better power (more).
If your powder load of choice blows the hammer back to half-cock, you have loaded waaay too strong/heavy a powder load, and should back it down immediately.
IIRC, if you install a T/C musket cap nipple in your Renegade, the interior end of the nipple will extend into the inside of the snail beside the patent breech and partially block flame travel until it's cut/filed off.
I've seasoned my T/C bores with Ox-Yoke Bore Butter, also lubing my boolits with the same, ever since it was first introduced - and rarely have to perform a deep cleaning on my BP rifles.
With the Bore Butter system, however, it's effectiveness is lost if "normal" cleaning methods or petroleum lubes are used.
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