If you're having problems with the o-ring blowing into the chamber due to using propane then you're better off just getting a new piston head. There are replacements (comes with the o-ring and everything) from Tanio Koba and Shooter's Design. The Shooter's Design is supposed to be very good as it's metal as opposed to the TK plastic.

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"Two Wongs do not make a Right"
XT Team - Task Force Wong; Tai-i Wong
-TFW Springer Weapon Assault Team (S.W.A.T.)