I'm talking about the one that replaces the bolt and carrier, it looks like it has a built in recoil spring?
So I'm wundering if I can make a pistol with a super short barrel, as there is no gas system needed, and no buffer tube?

Quoted: I'm talking about the one that replaces the bolt and carrier, it looks like it has a built in recoil spring? So I'm wundering if I can make a pistol with a super short barrel, as there is no gas system needed, and no buffer tube?

They use the buffer and spring to hold them in place but thats about it. As you indicated, the conversion units have their own recoil spring. Your pistol idea would work fine.

I thought there is a little square part that goes near the charging handle that hold it in place, therefore not touching the buffer, right?

On my Colt unit, the square part seems to be there to keep the unit from rotating.

To use one in a pistol, you would just need to plug the hole in the lower that the buffer tube threads into. I would find a plug that screwed in nearly flush with the back of the lower and then put a short piece of buffer spring and a small plug in there to keep the tension on the 22lr adapter. You could use the stock buffer retaining pin to keep everything from coming apart.

The Ciener kit does not require the buffer to hold it in place.

I've used mine many times with no buffer.