Airgunsonly.com
RECOIL,
MUZZLE JUMP and BARREL TIME _____________________________________________________________________________
The major European gunmakers have for many years recognized that the two largest contributors to inaccuracy in modern airguns is recoil [and its effect on muzzle-jump] and barrel time of the system. We can see many attempts to reduce these effects in the subtle design changes that are made year by year and model by model and of course the higher scores shot by the top marksmen are in some part due to these design changes. It is therefore important to recognize these new features and look for them in the new guns that you are buying.
RECOIL takes place in a firearm at the moment that combustion starts and chamber pressures start to increase. Recoil is a reaction to the firearm caused by the action of the combustion [action and reaction]. The pressure created in a forward direction is what causes the bullet to be driven out of the barrel. Pressures outwardly are absorbed by the walls of the action surrounding the chamber and are largely cancelled out. The rearward pressures are what cause the “kick” to be felt at your shoulder and are what cause the muzzle of the gun to jump upwards. It can be seen that the muzzle begins its upward jump before the bullet leaves the muzzle and therefore the projectile is affected differently each time a different amount of muzzle jump occurs. It can be easily seen, therefore, that any way that one can reduce the amount of muzzle jump will result in higher scores [all else being equal].
COMPENSATORS can reduce muzzle jump if they are designed correctly by exahusting the muzzle blast upwards and compensating the upward jump of the muzzle. This device was first used in rapid fire pistol with a great deal of success and it is now used in airguns. [see our article on compensators].
STABILIZERS are another compensating feature usually located in the bolt. It is a mobile weight mass, spring loaded and capable of absorbing some of the rearward recoil pressure and therefore reducing muzzle jump. This new system [at least in theory] seems to be effective.
STOCK AND GRIP DESIGN can affect muzzle jump by directing the rearward recoil backwards in a straight line with the barrel. Free pistols are regularly designed with a very low barrel line for this reason and we are seeing it in the newer air pistols. A very high buttplate position in prone rifle shooting is used largely to support the rifle at the shoulder directly behind the barrel centerline.
BARREL WEIGHTS or a heavy weight mass placed in a forward position on the gun can also reduce muzzle jump. The more moveable and adjustable that these weights are, the better.
LOW VELOCITIES decrease muzzle jump as is evidenced by the lower velocity of top grade .22lr. ammunition and the low [500 to 550 fps.] velocity of match airguns.
BARREL TIME is defined as the length
of time that the projectile remains in the barrel after the expansion of gasses
begin to the projectile forward. We know that the barrel begins to move [usually
upwards] from the moment that gas expansion starts. It therefore follows that
any way that one can reduce this barrel time will result in the
projectile being less affected by barrel movement
SHORT BARRELS will obviously reduce barrel time and this is the reason that match rifle barrels are made to be no longer than is absolutely necessary for the projectile to reach maximum velocity and stability. Air rifle barrels are often cut to about 20 inches for this reason [the new Steyr LG100 is only 17 inches]. The manufacturer has obviously proven that this is sufficient to produce top results. Of course we still require a much longer sight distance and therefore a barrel tube long enough to support a the front sight mount is installed……referred to as the “bloop tube”. The bloop tube’s only function is to carry the front sight, it does not, in itself, increase accuracy at all.
FASTER ACCELLERATION of the projectile in the barrel reduces barrel time. Obviously a “bigger charge” will accelerate the projectile faster but it has the very undesirable feature of creating more muzzle jump at the same time. In the case of precharged airguns [cylinder-air or CO2] the valving system becomes important and modern match airguns are now designed to introduce a maximum charge of gas very quickly. This can be illustrated by comparing them to the “high lift” and “long duration” valving systems used in racing engines. I believe that faster accelleration is the only real reason that cylinder-air is proving to be better than CO2. Air expands faster than CO2, the pellet accelerates faster and the barrel time is therefore reduced.