Try to get the extractor a bit more true. It is possible that the 2 ratchet teeth for those 2 chambers are a bit large and need adjusted. If the bolt drops and locks the cylinder up and the hand still has much ratchet tooth it has to force its way by to allow the trigger to finish its movement to release. In this case the extractor run out may be causing the rear cylinder to be force over just enough to cause the hand ratchet engagement to be a bit off. But the rear of the cyliinder is fairly well held in position by the ended of center pin engaging its location in recoil shield. But, then it don't take much