About WW2 fighter aircraft firing power

[Rob Philips]

Thanks, Harri, most interesting. Your material describes the Janecek invention, presented to the BSA company, who presented it to the Ministry of Aircraft Productions in Febr. 1941. The invention is a variation of the squeeze bore principle, designed to obtain higher muzzle velocities.

The basics are as follows. To drive a projectile to a very high speed, you need a lot of propellant. Using that, you also need a long barrel, so that the propellant can be burnt completely into propelling gases. Long barrels can become impractical, and this is where the invention kicks in. If you restrain a projectile, by narrowing the bore towards the muzzle, you create more time for the propellant to burn. This requires two things:
1. a tapered barrel, which is difficult to produce, and
2. squeezable projectiles, which are not too difficult to make.
The Janecek invention deals with the first requirement in a simple way: use a normal barrel, and add a choke to the muzzle. A relatively simple accessory, with which standard gun barrels can be transformed into high velocity ones. Or so the claim goes. Quite comparable to the choke applied to the muzzle of 12 gauge hunting guns.

Then the document continues, trying to sell the idea. For this a calculation of "hit probability" is introduced, and, as could be expected, this calculation demonstrates that hit probability is proportional to muzzle velocity. That's where the statement appears, that a 50% higher muzzle velocity leads to a hit probability that is five times higher. Elsewhere in the document the calculation leads to a factor eight. I believe that the exercize is executed quite well, but we must realize that this is a marketing exercize. The entire calculation is based on an assumed capability of the target to perform lateral movements, of deviating from a straight line of flight so as to avoid being hit. The outcome of the calculation shall depend on what is agreed about such lateral movement capabilities.

In any case, the British did not buy it. Neither the invention, nor its sales talk. I assume that the choke idea could not as easily be implemented to existing guns as was believed. We know that the squeeze bore principle was made operational and succesful by the Germans. They had to surmount the difficulties of producing a tapered bore to get the idea to work. They did so to obtain very high muzzle velocities, meaning very high kinetic energies, which were desirable for armour piercing applications. As far as I know, the Germans did not implement that technology in fighter aircraft armament, let alone that an increased hit probability would have been a reason for doing so.

Nevertheless, a most interesting find.

Regards,

Rob