Quite an interesting question. First let me say that I am experienced in machinining and toolmaking (a sub-discipline of machining), as well as being an amateur chemist and a student of industrial history, so I am well able to answer this question. The other answers really are guesses by non-experts and are incorrect in many of their details.

Possible with Some Caveats

It would be possible with some caveats. First, you would need to alter the weapon design to accommodate their abilities. Secondly, you would need to provide more than schematics; they would need to know certain processes and the steps to perform them as well as have plans for tooling needed to make the parts. However, if you gave detailed instructions for the different processes, it would be possible, given enough time and money.

Problem 1: Producing Tool Steel

The biggest problem would be getting tool steel. Tool steel is not needed for the gun itself but is needed for making the tools to make the gun, the main thing being files. Once you can get them making a good file, that is 90% of the problem. You may think it funny, that something as simple as a file is the key, but that is the truth. Tool steel requires manganese, chromium and molybdenum, so you would need to help them locate, identify and purify those metals.

Problem 2: Measurements

As long as you provided an example weapon or EXTREMELY detailed drawings they could measure everything with calipers, but it would be very, very tedious. If you provided them with the key to making a casting metal, such as cerrosafe that would make it much easier. To make this they will need bismuth and antimony, which once again, would have to be located, identified and purified.

Problem 3: Propellant

For the ammunition they would need to make a corned propellant and primer. This requires pure sulfuric acid, nitric acid and various other chemicals. They would need to be taught how to make these chemicals. Without high quality glassware and catalysts it would be possible, but very tedious to do this.

Problem 4: Springs

To make the springs they will need spring steel, wire dies and a winding machine. Since we already have manganese and tool steel from Problem 1, this would not be too hard.

Problem 5: Cartridge Casings

Modern cartridge casings are forged using complex dies. Making these dies would be extremely tedious and difficult for them. As an alternative, you could make the cartridges by silver soldering them from hammered sheet brass, much easier.

Problem 6: Boring and Forging

In order to make the key parts of the weapon, the bore, the frame and the receiver, they would have to acquire a range of specialized tools for forging, boring and rifling these parts. Assuming they have tool steel and you have provided them with plans and instructions for creating these tools, this would be possible for them to do given enough time and manpower.

Problem 7: The Screws

A normal gun uses screws to hold many of the parts together. In theory they could hand file all the screws from steel rod. This would be INSANELY time consuming. Alternatively, you could teach them to how to build a lathe with lead screw and gearing. Once the lead screw is filed, that is the basis for making any type of screw given the proper gears. This would probably be the best option. A third possibility would be to use rivets instead of screws, however, since the lathe would be useful for other parts it would make sense to build the lathe.

Quite an interesting question. First let me say that I am experienced in machinining and toolmaking (a sub-discipline of machining), as well as being an amateur chemist and a student of industrial history, so I am well able to answer this question. The other answers really are guesses by non-experts and are incorrect in many of their details.

Possible with Some Caveats

It would be possible with some caveats. First, you would need to alter the weapon design to accommodate their abilities. Secondly, you would need to provide more than schematics; they would need to know certain processes and the steps to perform them as well as have plans for tooling needed to make the parts. However, if you gave detailed instructions for the different processes, it would be possible, given enough time and money.

Problem 1: Producing Tool Steel

The biggest problem would be getting tool steel. Tool steel is not needed for the gun itself but is needed for making the tools to make the gun, the main thing being files. Once you can get them making a good file, that is 90% of the problem. You may think it funny, that something as simple as a file is the key, but that is the truth. Tool steel requires manganese, chromium and molybdenum, so you would need to help them locate, identify and purify those metals.

Problem 2: Measurements

As long as you provided an example weapon or EXTREMELY detailed drawings they could measure everything with calipers, but it would be very, very tedious. If you provided them with the key to making a casting metal, such as cerrosafe that would make it much easier. To make this they will need bismuth and antimony, which once again, would have to be located, identified and purified.

Problem 3: Propellant

For the ammunition they would need to make a corned propellant and primer. This requires pure sulfuric acid, nitric acid and various other chemicals. They would need to be taught how to make these chemicals. Without high quality glassware and catalysts it would be possible, but very tedious to do this.

Problem 4: Springs

To make the springs they will need spring steel, wire dies and a winding machine. Since we already have manganese and tool steel from Problem 1, this would not be too hard.

Problem 5: Cartridge Casings

Modern cartridge casings are forged using complex dies. Making these dies would be extremely tedious and difficult for them. As an alternative, you could make the cartridges by silver soldering them from hammered sheet bronze, much easier.

Problem 6: Boring and Forging

In order to make the key parts of the weapon, the bore, the frame and the receiver, they would have to acquire a range of specialized tools for forging, boring and rifling these parts. Assuming they have tool steel and you have provided them with plans and instructions for creating these tools, this would be possible for them to do given enough time and manpower.

Problem 7: The Screws

A normal gun uses screws to hold many of the parts together. In theory they could hand file all the screws from steel rod. This would be INSANELY time consuming. Alternatively, you could teach them to how to build a lathe with lead screw and gearing. Once the lead screw is filed, that is the basis for making any type of screw given the proper gears. This would probably be the best option. A third possibility would be to use rivets instead of screws, however, since the lathe would be useful for other parts it would make sense to build the lathe.