I was always led to believe they start at the front, hence the need to front-fill with Vaseline or similar.

Most of my cap'n'ball shooting has been with an 1856 3rd Model Tranter, and I would NEVER forego the Vaseline seal. Might be possible to go buy another Uberti, Pietta,
or Ruger Old Army at the local gunshop, but wrecking an original Tranter would be unforgivable.

One year I came second in the black powder aggregate at the Arafura Pistol Club with that Tranter, top-scoring over all those modern spaghetti guns and stainless Rugers.
Beaten only by a Webley .22 single-shot with modified chamber insert! Not technically cheating by the letter of the law, but hardly in the spirit of the event!
Some R-soles will go to any lengths to win...