How did they film the gun spinning scene?

Look at the gun. Look at how:

• It is spinning while staying at the exact same place of the table. Even in the third spin, when Walter doesn't even look at it, it does not skip, it does not slide sideways even a bit.
• It is spinning around a point somewhere behind the drum. A point at which the gun shouldn't even be touching the table, let alone it being the center of mass, allowing for such an in-place spin.

A gun lying on the table, and being spun, would not behave like that.

Which means the gun is not lying on the table, but is mounted in some way -- making it rather easy to make it end up pointing in any desired direction.

Affixing a mount to a revolver is easiest done not at the drum, but at a point behind it (where you could e.g. drill a hole into the gun and affix a rod to that hole) -- explaining why the gun spins so unnaturally.

There's a bunch of ways to do this.

Most likely:

• They took 130 takes to get it right

Less likely:

• There's an invisible turn table under the gun
• They used an electromagnet to stop the gun
• The gun is on a clear plastic stick and someone out of sight is turning it

Any answer given is likely to be speculation, but if it were me I would just do one long take until I had all the material I needed.

There's a cutaway to Walt's face in the middle of the sequence, splitting it into two shots. This provides a convenient edit point so all the editor needs is one shot of the gun pointing at the plant, and one shot of the gun pointing at Walt twice in a row. Then take the two bits of film and edit them together so it looks like they happen in sequence.

There is something spinning under the gun, it looks like it is in the table, when i saw that 2 mins ago (:D) i came here, at the scene where the camera was moving, they had edited that 'thing' out. But i knew it was for making the gun stop.

I don't know for certain but given computer technology in the 2010s and the cost of computing power vs the crew cost of filming many times, it seems likely that the gun was rendered. The fact that a rectangle can be drawn around the gun spinning, completely separating it from the rest of the frame makes it even more likely that it was isolated and done by computer. If the CG element doesn't actually interact with the rest of the scene it's going to be easier and cheaper.

Now if--on the other hand--Walt were shooting the scene, he could easily hide a small electromagnet (perhaps removed from a toaster with an electric release?) that he triggers remotely from a button in his hand (modified garage door opener?) as the gun slows down. He'd need two for the two stopping positions, and he'd have to push the button at just the right moment.