Rarest guns are the 12 original P5 prototypes given to the Jax Warriors in 1991. These were the precursors to the xvalve and the first guns to run exclusively off compressed air.
Besides those the rarest parts are the titanium valves we had custom made. They turned out to be almost the same weight as stainless and we think they got mixed in with the regular valves and went out the door in guns. None have ever been found but then again you may have one and not know it.