In the episode North Star, Trip, T'Pol and Archer all beam down to the surface of a planet with humans from the American wild west. Trip has a period appropriate gun. Where does he get it from?
There are several easy to consider choices to explain how Trip ended up with a firearm during the episode in question:
Trip is a gun enthusiast and has a personal firearm created on Earth (or some other planet) with the ability to create such technology using standard mechanical means. Likely the easiest explanation, since Trip made regular visits to Earth. Making ammunition should be able to be made using materials found on Enterprise.
Using the Enterprise NX-01 transporter, the crew could have beamed a local handgun from the environment to aid in appearing authentic. Since there were no replicators available to the Enterprise (ENT) crew, this makes perfectly good sense for them to have stolen clothing complete with firearms from an unsuspecting homeowner (all done off-camera).
- The Enterprise could have, what would be high technology to us, but commonplace to them the capability to have created the gun using 3D printing technology. Seeing the Enterprise is an environment needing sophisticated tools and manufacturing, it makes sense for them to utilize a 3D printer to create mechanical devices as needed. As long as the design blueprints are available any reasonable handgun-like device with mechanical components should be able to be created.
World's first fully 3D printed handgun, 2013
He probably made it, I forget which episode it was when Trip, Reed and other engineering and armory personal built several phase cannons from scratch.