So I was laying back watching Castle on TV (S03E04 "Punked" for the curious) and one of the plot points was that two steampunkists had a duel with black powder dueling pistols.
Not to give away too much, but the defining clue was that the ballistics on the fatal ball didn't match the ballistics of either pistol.
Q. Will a cloth patched ball in a pistol have repeatable ballistic markers, or was it just more 'tv writer's logic'? (From the 25yd lack of accuracy, it would appear they were smooth bore.)
What is meant by "ballistics?" It seems to me from your description that three pistols were involved; the two held by the duelists, and a third used by the third actual murderer.
What if the 3rd pistol was a different caliber? That is a dead giveaway that neither duelists' pistol could have fired the fatal round.
Do you mean a third gun charged with a different amount of powder? I'm not sure how you would really determine this without having all three guns and running some very intricate tests and even those might not help.
I have recovered modern .38SP wadcutters that were all fired into a similar backstop from the same distance .... and some were near pristine while others were badly mushroomed.
A bullet that hits a person and hits bone might wind up in worse condition than one that only penetrated soft tissue. In the Civil War, soft lead balls would often shatter upon hitting bone, shatter the bone as well, and cause horrific wounds.
I think your TV sleuth over-analyzed the situation, but without more knowledge from the TV show I can't really say .... but I will say one thing;
If you're watching TV, suspend your disbelief, sit back and enjoy the ride, if you're not enjoying the ride, change the channels, if there's nothing there, turn off the TV and read a good book. Good books are easier to find than good TV shows.