[Celjabba]

The question have been debated to death (and ressurected for further debating) in the 1632 series research groups (and also in the "Island in the sea of time", in universe).
Although the discussion are aimed at "best guns to equip a (large) army".

As I am not a gun aficianodo, I can't remember the conclusions, but it should be a good search start.

To equip smaller groups, I imagine the answers are something else.

My guess, looking for mass-produced mid/late 19th century military guns should give some good answers.

I think the hardest will be to find the schematics.

Also, tell your scavenger to search for a decent milling machine, along with tungsten carbide bits and tooling !