c-g-designs

CRATERING

          Cratering can be qualified as a form of extrusion allowing the metal of the primer cup to flow around the firing pin tip in the firing pin hole of the boltface. This is a form of impact extrusion as know in metalworking.

          The principal cause is overpressure, but overpressure is most often one of the primary causes only, aggravated by other factors:

          For the metal flowing to occur, there must be room for the metal to flow, and the possible additional causes  are:

          1°)-The relation between FP tip diameter and its hole in the bolt. The eventual gap between those diameters allow the primer cup metal to flow to fill the gap.

          In this aspect, the smaller firing pin diameter is at advantage, as its frontal surface(the half sphere) receive less blow from the pressure developped.

          Also, the smaller clearance does not leave room for the metal flow to occur.

          It is no doubt that, everything else equal, a small FP tip with a well-sized hole does not crater easily.

          2°) A ”spongy’ FP blow. In this case, when pressure builds up, the FP moves backwards (rebound) the metal flowing process can start. Again here,the smaller FP tip is at advantage, as its smaller frontal surface does receive less impact and resist better to the blow.

          3°) Primers too deeply seated. This allow the process to start by making the primer to move backwards unsupported by the boltface bottom….and the processus continue to cratering.

          4°) Primer too loose in the pocket. Under FP impact, primer moves forward and the shock in return allow the flowing processus to happen as well.

It is to be noted that the loose primers also is the cause of circular pitting of the bolt face.

          Nothing is written on stone in this, for instance, a too strong FP spring, mostly if protrusion is set to maximum, can cause the primer cup to deform too much, and then, the process start again but, this time, as the FP tip cannot moves back, it support all the pressure rise…and can bent or finally break if not first quality. This situation also is a primary cause for primer piercing. Resetting protrusion to normal can resolve this point.

          The fact the FP system is maintained cocked by the sear  from the bottom makes that, in turn,in the one-piece FP system, if there is play at FP tip level in its hole, the FP tip tend to be exventered in the said hole in lower position, and risk to impact off-center. Bushing is the common solution to reduce this cpnsiderably.

          The above cannot happen with a floating firing pin tip, always perfectly centered.

          R.G.C 

         11/2013                                                                                                        12/2013