So my M125 is broke >:(

[Nathan]

Not too sure how I did it, but I did. I was shooting the .25 M125 and it didn’t seem any different than before in terms of firing behavior. Don’t know why I decided to chrono check it. So the first shot was a Crow Mag (~26 gr.) and the reading was 444 fps. What?! Thinking it was a bad reading I shot another, 448. So I tried an Air Arms pellet. 452 fps. Thinking maybe my chrono was on the fritz, I grabbed a different gun. I loaded up the .22  Hatsan M135 with a CPHP. 952 fps. Next shot was 947 fps. Well, I figure the chrono is fine. So I started tearing into the 125.
So after wrestling with it in the spring compressor and ( I hate to admit) having trouble remembering how it comes apart, I finally got it disassembled. As soon as I pulled the piston I knew what the problem was. See for your self.
The seal is the Macarri seal I put in. Not too sure if I'll order another Macarri ( I really like his stuff) or if I'll try a Vortek one. The main spring looks fine, not even canted, so I think I'll reuse it.
It will be a few days or longer  till I get the new seal and get it back together. I'll post how it turns out.

Nathan

[BenjiHunter]

If a piece fall off like that, the seal material is probably too hard.

[Airgun Van1]

My 125 25 did the same thing I replace the Piston seal about 4 months ago and now all of the sudden it will not cock

[Privateer]

That looks more like a burn out.
Also seems to be in line with the cocking slot?
Is the grey stuff moly?

[Nathan]

I don't think it's moly. I'm not sure why it looks so "dirty" in the picture. I hadn't really noticed.

Nathan

[Privateer]

The pictures look like there may have been a slight nick near the cocking slot.
The Grey stuff looks like maybe some moly got into the compression area.
Add pressure and you get dieseling.
The temps and pressure do get high enough to follow any nick and cause a burn out like that.

I also see several other areas that show signs of nicks.
Maybe not on the seal lip but they are there in the 1st picture.