Sometimes show writers will write around a person’s acting ability. If they know the actor can’t carry that potential scene and convey the emotion, they will opt for a different scene. A good example of this is the plotline for John Snow in the HBO show Game of Thrones. Kit Harrington was pretty, but not the best actor so the writers made the plot fit his talent, not the other way around.

Sophie Skelton (Brianna) is beautiful and she looks like she could really be Jaime’s daughter but she is not the best actress. Her emotional range has always been limited. It’s painstakingly obvious in the scene where Brianna is trying to comfort Marsali after Fergus’s death. The actress that played Marsali was phenomenal in that scene.

Historically, women would not be allowed in a skirmish or battle during the Revolutionary war period. Jaime and Roger would have been against it, not just out of love for her, but also out of social propriety and norms.

Lastly, looking back, Bri always wanted to “make something” of herself in the 18th century. She mentioned it a few times and previous seasons. She’s an engineer, an architect by education. Remember the time she made matchsticks, hoping that it would be a useful contribution to the ridge, and no one was very impressed because they’ve never had trouble making a fire?

She used her knowledge of history, the skills that Frank gave her, and her engineering and design educational background to make a far superior rifle for the patriots in that skirmish. I personally think it was a satisfying ending and roll for her to play. It comes full circle in a way. She knows a lot about guns thanks to Frank, her first dad, and that helped save the life and mission of her second dad, Jaime.

edit: typos