SWAPPING TO SYNTHETIC SEALS REDUCED AIR FLOW all) older airguns with leather piston seals had relatively short piston strokes in the region of 65mm - 70mm, and it was a combination of a short stroke and auto-ignition that favoured wide transfer ports, as Mike Wright discovered. As most people know, the flow of air through the transfer port chokes at the localised speed of sound when the pressure downstream is half that of upstream. In a springer that is not dieseling, or not reliant on dieseling to achieve muzzle energy, the ideal time for the flow to choke is when, or very shortly after, the piston reaches the end of its compression stroke, when temperature and pressure are at their peak, and before the piston retreats up the cylinder. In springers that are reliant on dieseling, the temperature and pressure peak caused by the diesel advances the onset of transfer port choking to well before piston bounce, reducing the air flow through the port at the very time that the piston should be contributing to the acceleration of the pellet, and lowering muzzle energy below its potential. The speed of mass air flow through the transfer port at any point in the piston stroke is governed by the diameter of the transfer port. For any given cylinder pressure, the narrower the port, the higher the mass airflow rate. To understand why, consider a hose pipe with water flowing through it; if you squeeze the end oil worked well, but if the muzzle energy was raised to the point at which cylinder temperature exceeded 442ºC, it combusted (dieseled), and repeated dieseling shots did the leather seal no good at all though, to be fair, it took a lot of shots to damage the seal to the point at which it failed to do its job. TRANSFER PORTS Many of the airguns originally fitted with leather piston seals relied upon dieseling to achieve their target muzzle energy, and the fact that this was intentionally designed into the rifle is evident in the diameter of their transfer ports. Many rifles with leather piston seals had transfer ports that were 4mm or so in diameter, and in the absence of any scientific idea at the time of how airguns worked, that figure would have been arrived at by the manufacturers by old-fashioned trial and error as giving the highest muzzle energy. In addition to wide transfer ports, most (not » 73 www.airgunshooting.co.uk AIRGUN WORLD
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