The .22LR rimfire cartridge was originally developed as a black powder cartridge late in the 19th century. Even when filled up with black powder, it was not a high pressure cartridge, so the rifles built to shoot it could be cheaply made. As the use of black powder generally gave way to smokeless around the turn of the 20th century, so smokeless powder started to be used in the .22LR cartridge as well. Now smokeless powder has twice the energy density of black, so to keep the pressures the same the case could only be half filled with smokeless powder. This means that when the cartridge is in the chamber, the powder is laying in the bottom half of the case.
The firing pins on rimfire actions almost ubiquitously strike the rim at the 12 o'clock position. This is because there is usually a slot on the bottom of the bolt to allow it to run over the ejector pin. This slot of necessity ingresses into the area of the bolt face where the firing pin would be if it was at 6 o'clock, so from a design consideration, the firing pin is usually located opposite this slot, in the 12 o'clock position.
However, anyone who has lit a bonfire will know it is usually better to put a match to it at the bottom rather than light some twigs at the top. So, the argument goes, it would be 'better' if the firing pin struck the rim at the bottom, where the powder was, rather than at the top, where it is not. This would lead to more consistent burning of the powder, which would result in better accuracy. The argument has sufficient appeal that a number of custom rimfire action makers now offer 6 o'clock firing pin position bolts, and at least one European match rifle manufacturer is now making its bolts with the firing pin at 6 o'clock as a default.
But is it really 'better' to have the firing pin at 6 o'clock? Does it matter where on the rim the primer is ignited? Surely, once the primer starts explode, it lights up evenly and pretty much instantaneously all around the rim? As with much about rimfire ballistics, there are many strong opinions on the matter but little hard fact to back them up. This experiment was an attempt to shed light without generating (too much) heat.
We should expect to see a reduction in the shot-to-shot variation in chamber pressure though. Accordingly, the Baikal test action and a clean test barrel with a cone breach was set up in the Border Barrels Recoil Pressure Gun to measure the peak chamber pressures. A Recoil Pressure Gun uses Newton's third law, which states that the force making the gun recoil backwards is equal and opposite to the force accelerating the bullet up the barrel. The force pushing the bullet up the barrel is just the chamber pressure times the area of the bore, so in principle it is possible to deduce the chamber pressure quite directly by measuring the recoil acceleration of the gun. Given the recoil acceleration, the recoiling mass is also required, along with the bore diameter and the masses of the bullet and powder in order to compute the chamber pressure. These four variables are dialed into the box on the bottom right hand corner of the bench as seen in the photo above. The accelerometer is attached to the back of the carriage and is attached to the RPG electronics via the twisted pair which can be seen connecting the carriage to the box. An Oehler 35 chronograph with sky screens centred at about 5.5 yards from the muzzle was also set up to measure bullet velocities. Eley Tenex batch 1010-05045-1045 was used for this test.
Initial pressure results from the Recoil Pressure Gun were disappointing as there was a large variance in pressure. The recoiling mass of the carriage + action + barrel was over 22 lbs and so the consequentially small recoil acceleration due to the puny rimfire cartridge was being significantly affected by the static friction of the recoiling carriage on its rails. By sliding the carriage back with one hand while pulling the trigger with the other, the static friction was overcome and results became much more consistent.
There has been some mis-understanding about this seemingly primitive, subjective and irreproducible technique "sliding the carriage back with one hand while pulling the trigger with the other." It should be explained that there are two kinds of friction. Static friction and dynamic friction. Anyone who has ever pushed a large packing case across a floor will know that it takes a lot of effort to get the box moving, but once it is on the move it is much easier to keep it moving. The force to overcome the static friction is much higher than friction between the box and the floor once the box is moving. The point to note is that it does not matter (within reasonable limits) how fast the box is moving across the floor, the frictional force will be pretty constant. So it is with the recoiling carriage on its rails.
The idea is to get the carriage moving on the rails so that when the cartridge explodes and the carriage starts to recoil backwards, it does not have to overcome the static friction before it can start to move. Once the carriage is moving, the dynamic friction is much lower than the static friction, so will soak up much less of the recoil force, and it does not matter how fast I am pulling the carriage back so long as it is moving. Any acceleration I may be giving to the carriage will be thousands of times less than the measured acceleration the exploding cartridge is imparting to the carriage. So, this seemingly primitive, subjective and irreproducible technique is actually nothing of the sort.
The action was then rotated by loosening off the lock-ring holding it onto the barrel, turning the action upside-down and tightening up the lock-ring. The firing pin now struck the rim of the cartridge at 6 o'clock. The barrel was not rotated and there was no other change made to the apparatus. 28 shots were then fired. The mean pressure was 1135 bar with a standard deviation of 194 bar. The mean measured velocity was 1030 ft/sec. with a standard deviation of 8.8 ft/sec.
The action was then put the right way up again (firing pin at 12 o'clock) and another 33 shots fired. The mean pressure was 1292 bar with a standard deviation of 286 bar. It was noted that the velocities had been gradually creeping up and this was attributed to the sun coming round past the shade set up to protect the sky screens from direct sunlight, which affected the 'seeing' of the bullet by the photodiode in the sky screen. No more velocities were taken.
The action was then put upside down again (6 o'clock firing pin) and another 27 shots were fired. The mean pressure was 1193 bar with a standard deviation of 203 bar.
For the next three runs, the mean pressures were much more in line with what would expected for the .22LR rimfire cartridge. By far the most interesting result is the reduction in the standard deviation on the pressures with the firing pin at 6 o'clock. For the second run with the firing pin at 6 o'clock, the standard deviation on the pressure was less than half that for the first run with the firing pin at 12 o'clock. As mentioned above, the magnitude of the difference in these results may an anomaly, but the trend is certainly indicative. The pressure SD for the second and forth runs (6 o'clock firing pin) were a third less than for the third run, where the firing pin was at 12 o'clock.
It is interesting to note that the mean pressure was lowest for the two runs with the firing pin at 6 o'clock. Now this is pure speculation, but it may be that a comparision with experience with centerfire cartridges is justified here. It is well known that if the cartidge is "loaded down" so that the case is only partly filled, then partial detonation of the powder can occur. The high pressure experienced by the powder as it is thrown forward by the primer explosion and crushed up against the back of the bullet can cause the powder to partially detonate - a phenomenom I have often seen when looking at the pressure-time output (for centerfire cartridges) from the Recoil Pressure Gun on an oscilloscope. With slow powders in large magnum cartridges, this detonation can be particularly dangerous and blow the gun up. See the "Handbook for Shooters & Reloaders" by P. O. Ackley for a description of this phenomenon. It may be that a similar effect is at work in the rimfire cartridge, where the powder is not being efficiently ignited by the primer but being blown forward against the back of the bullet where it partially detonates. This would result in a raising of the peak pressures observed.
The mean velocity for the second run with the firing pin at 6 o'clock was slightly higher than for the first run (firing pin at 12 o'clock) and the SD was slightly lower. It might be argued that if the powder is burning more consistently, and so more efficiently, the velocity might in consequence be higher and the SD lower, particularly if significant fractions of powder are remaining un-burnt in the 12 o'clock position but are getting burnt in the 6 o'clock position. A well known problem of rimfire tunnel ranges is the accumulation of unburnt powder in the tunnel. There have been several explosions as the carpet of unburnt powder finally sets light! But the differences here are not high enough to be statistically significant, given the number of shots fired, and so this result should not be given too much weight.
So, if more consistent burning of the powder does not improve the velocity spread, in what way can it be of benefit to accuracy? The answer is that it is the recoil force of the rifle that excites the vibrations in the rifle barrel, and it is these vibrations that affect where the barrel is pointing when the bullet exits the muzzle. The recoil force depends upon the chamber pressure, so if the variations in chamber pressure can be minimized, then variations in the way the barrel vibrates can be minimized as well. The consequence of using a 6 o'clock firing pin rather than a 12 o'clock firing pin - all other matters being equal - would probably be slightly 'flatter' groups with less vertical spread.
Although the powder would appear to be burning more consistently with a 6 o'clock firing pin, this does not appear to lead to lower spreads on the muzzle velocity, nor should this be expected.