Interesting update it looks like Diana have done something
QUOTE from the DWC today.
Umarex sent me a TO5 piston, I didn't figure that out until the rifle wouldn't fire. So I had to call again and get the TO6 piston ordered. It came yesterday so the rifle is back together and shooting. A couple things to note is that there ia another row of crimps closer to the seal end and the three crimps in the original location are about .020" deeper than on the piston that came in the gun. So it appears that Diana recognized there is an issue and have taken steps to correct it. QUOTE.