I don't think it is your 4F priming powder my 4F powder has to be5 years old think I still have a little less then 1/2 the can left.I would drill out your vent liner I here you say you use a vent nipple tool to clean out you vent liner sounds like the old t/c vent liners with the screw driver slot hole.The new improve vent liners have to be .070 .I would think take a old vent liner and open it up to .060 I use to drill out the t/c but I don't remember the drill size :another important thing to view is you vent liner is not below the priming powder pan and after firing the gun look and see if there is no smoke coming from the bottom of the trigger guard seen vent liners that are below the lock crack and split the stock in the lock area to to ignition under the lock itself.