I'm sure that anyone who has built a FAL before is familiar with this method, but I had a member ask about how to do this in another thread.
First the tools.
This can be done completely with just a hacksaw and a large chisel or even a Dremel, if you have plenty of the HD cutoff wheels.
But a 4" cutoff wheel makes it a breeze.
You want to cut (carefully, your barrel threads are right under this cut) until you can just barely see the very edge of the barrel threads. Do the last part of the cut by hand using a hacksaw, if you are using a cutoff wheel.
Insert your biggest chisel in the slot and give it a good whack. It will split right along the cut. this will relieve the tension on the barrel allowing you to spin the stub right off.
If you look close at the cut, you can see the jagged edge where it split under the cut.