Derya Mk12 stuck gas piston

Use WD40 or penetrating oil and soak the piston. Wrap the end of a large crescent wrench with tape to pad, not mar the piston finish, open it just bigger than the barrel. give it a few good whacks on wrench handle with a hammer. When you finally get it loose, go at the barrel, inside of piston and piston shroud with a dremel buffing wheel. It will loosen up with breaking in and wear. Don’t put lube in the piston area, it’s what causes the seize up/gunk. Clean piston frequently during break in, I had same issue and it was made worse by the chrome coating coming off from inside the piston shroud and piston, bits would get mixed with carbon and seize up.

It runs like a champ now after some wear.
 
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