Look at the video from this company, this gun is exactly how it is in front.
Look at the front of the barrel, it has a metal stem (photo 1) that pushes the base of the silicone cartridge. In front of the metal, I had a plastic piece that had the screw embedded, so I couldn't unscrew it. This plastic piece had a diameter almost the same as the diameter of the aluminum tube, so when the cartridge was inserted, the stem had the same diameter as the entire cartridge, so HOW could it push the base? Because I had never had such a gun before (I had an old cast one), I put them side by side and that's how I realized the mistake. I cut the plastic (photo 2) to the diameter it should be, that is, the same as the old one (photo 3), and now it can be inserted to push the base of the cartridge (photo 4) and start dispensing the silicone (photo 5).
If I found a way to remove this plastic, the axis would remain exposed, so every time I pulled the trigger, the pressure would be in the center of the base of the silicone like a skewer, and not uniform, so the job wouldn't be done correctly.
With a diameter almost the same as the aluminum tube, even a click larger than the cartridge itself, then HOW would it push the base?
Until I found the trick, I thought about throwing it away twice (I couldn't return it because it was scratched), but in the end, I found a solution.