They have the die.
If you have a lathe you can chuck the barrel in the headstock along with a steady rest about half way down the barrel and center the die on the tailstock.
Bring the tailstock to bear on the muzzle then back it off and install the die and reset the tailstock.
Hand turn the headstock until the die begins to cut, then set your speed and turn on the lathe and feed the die onto the barrel.
Don't have a lathe?
You can install the barrel between a long clamp using a bore mandrel.
Use the clamp to center the die against the muzzle, then hand turn using a die wrench.
The advantage of a die over a lathe cut thread is that you will obtain much cleaner and more uniform threads using a die.