I've had a quick read and this did stick out:

.... and a gunsmith told me that the barrel had become loose and the silencer was bent out of true.

Now I am not familiar with the HW so pardon my ignorance on the finer details. My first thoughts are did you buy the gun new?
Reason is this gun might have been dropped and damaged, but by a bit of fettling by the previous owner (such as loosening the barrel) was restored to firing great. For a while. Until maybe the barrel got a bit too loose where re- tightening put it back to the broken state.
If we rule the barrel out then I think the only other possibility is pellet damage either due to some misalighment in the breech or pellet probe. Again this loose barrel seems to stick out a bit (as does bend in the moderator - how?).
Is it possible to manually load a pellet into the breech and push it into position without using the probe on the bolt? maybe a matchstick? the you could feel if there was a step that would damage the pellet.
Finally - a bit of lead stuck in the rifling which won't budge!
Hope you get it fixed
atb
Simes