Firing Pin Diameters

[ovenpaa]

Robert Chombart and I have been discussing firing pin diameters, something that should be of great interest to those shooters moving towards the .308 Bolt Face cartridges with small rifle primers such as the .308 Lapua Palma and the 6,5x47 and this is Robert's observations on the subject which I found very interesting:

ABOUT THE QUESTION OF FIRING PIN DIAMETER:

A .0625 FP tip requires over 35% LESS impact energy than a .080 dia. One half-sphere surface = punched surface). The differences in protrusions increase this difference. If we knew the mechanical properties and thickness of the primer cup, It would be easy to calculate the impact energy needed to ensure both FP diameters indentations, bearing also into account the slight protrusion difference between both tips (.080 = 1,6mm , .0625 = 1,4mm protrusions).

Not boring anyone with other concept considerations, it is my belief a reduction of FP diameter should go hand in hand with impact energy reduction.

.0625” Pin:
Cylindrical metal displacement (protrusion 1,4 less r=0,6mm (0,6 x 1,6 x 3,14) = 3,01mm²
Hemispheric displacement Ø1,6 x 1,6 x 3,14 / 2) = 4,02mm²
Total for 1,4mm (.055) protrusion = 7,31mm² (.0113 sq/inch) metal deformed.

.0800” Pin:

Cylindrical metal displacement (protrusion 2Ø less r = 2–1 x 3,14 = 3,14mm²
Hemispheric metal displacement 2 x 2 x 3,14 / 2 = 6,28mm²
Total for 1,6mm protrusion = 3,14 + 6,28 = 9,42mm² (.0146sq/inch) deformed metal.

Assuming the primer cup thickness being a constant, it is then to conclude that the smaller firing pin diameter needs only 2/3 of the impact energy required for the larger. I would have gone further in determining the total impact force needed for the punching, but have varying indications on cup thickness's, none on alloy and hardness. Also to consider is that LR primers could have usually thicker bottoms than SR.

A very rough maximal estimate, (assuming primer cup thickness of 0,5mm and CuZn30 metal in H11 or H12 condition) would give some 95 Newton for the 2mm and 60 Newton for the 1,6mm.

This might be buggering flies in some sense, but the difference is interesting to bear in mind.

R.G Chombart

One of the advantages of using a smaller diameter firing pin is less spring force is required which would give a reduction in the effort required to lift the bolt, which means less chance of disrupting the alignment of the rifle on the firing point. of course simply reducing the diameter of the firing pin tip is not enough as the bolt face would either need to be bushed to suit the reduced diameter.

[northumbrian]

Not to mention the increase in component longevity, with reduced forces acting upon them.
A very interesting observation, thanks for sharing it.

[R.G.C]

Not to mention the increase in component longevity, with reduced forces acting upon them.
A very interesting observation, thanks for sharing it.

Absoluteiy. The subhect is vast and only approached above.

To mention only is the important reduction in cratering,but this is an other subject.

R.G.C

[saddler]

One of mine is a flat pin

...its also missing from the gun altogether!