Gun Choke

More, much more, than you ever need to know about choke
By Wally Albert - Gunsmith

Choke is the constriction in diameter of the bore at the muzzle end of a shotgun barrel. Its function is to reduce the spread of the shot as it leaves the muzzle of the barrel.The shape of a typical choke is shown in Fig. 1. Note that the angle and length of the tapered section, its surface roughness and the length of the parallel section vary from manufacturer to manufacturer. The greater the degree of choke, the tighter the pattern; that is, the less the diameter of the spread of the shot.

Figure 1: Schematic section of a choked muzzle. Note the parallel section following the constriction

Choke was invented in America by a country gunsmith in the second half of the 19th century. W.W. Greener, a famous English Gunmaker, became aware of the invention and took the idea to England where he developed it. He started to evaluate the effect of the choked barrel by firing at steel plates located at 20 to 25 yards from the shooters front foot, which was the distance that most game was shot at in England. However, he found that the resulting patterns were too dense to clearly show up the small differences in the construction of the choke with the result that, by trial and error, it was decided that 40 yards was the ideal range at which to pattern the gun, and that the percentage shot strike within a 30 inch diameter circle was the best means of evaluating the effectiveness of the choke itself.

It was finally determined that, irrespective of the bore of the gun, that with the paper cased cartridge of the day with a card over shot wad, that .040 inch (or 40 points of choke) could put 70% of the pellets inside the 30 inch circle at 40 yards; more constriction did not increase the pellet count in the circle. Hence this was called a "Full" Choke,

A cylinder bored barrel (no choke) was found to put on average 40% of the pellets into the 30" circle.The complete set of results obtained were as follows :

Type of boring	"Points" of choke	Percentage of Pellets* in a 30" circle at 40 yards
Full Three quarter Half Quarter Improved cylinder True	40 30 20 10 3 to 5 NIL	70 65 60 55 50 40

*Note: These percentages were for the then current cartridges with felt powder wads and cardboard over shot wads. Introduction of the folded crimp case improved patterns by an addition 5%; introduction of the plastic cup wad improved patterns by another additional 5%. So today, using a modem plastic cartridge case/shot wad combination cartridge a "full" choked gun (with 40 points of choke) will put about 80% of the shot in a 30 inch circle at 40 yards, effectively increasing the range of the gun by 10 yards.

American choke terminology, based on plastic shot shells, is different from the English as follows:

English	American
Full, at 40 yds Three quarter Half Quarter Improved cylinder Cylinder	Full, at 50 yds Improved modified Modified Improved cylinder Skeet Cylinder

Use of Choke

The effective use of choke is illustrated in Fig. 2. The inner cone is formed by the muzzle at one end (the point of the cone) and presumably a 30 inch circle at, in this case, 50 yards. The outer cones start again at the muzzle, but terminate at 30 inch circles located at specific shooting distances. This diagram is not strictly correct, the cones are in fact slightly trumpet shaped, but the effect is small except that the more open chokes slightly concentrate the shot charges at the closer ranges. If we consider the cylinder choke at 40 yards only 40% of the shot will hit within the 30 inch circle, but the density of the pattern would be inadequate for clean kills. However, at 20 yards range 80% of the shot would be within the 30 inch circle and the pattern density would be adequate for clean kills and the total area of coverage would accommodate normal aiming errors at such short range. In practise cylinder bore is almost never used, 3 to 5 points of choke giving more controlled patterns.

The same logic applies to the use of the other chokes; think in terms of 30 inch circles at expected range, as this will be where the bulk of the charge will be, and select the choke accordingly. When shooting at game the shot size is selected on the basis of pellet energy required to ensue adequate penetration and the weight of the shot charge is selected on the basis of the number of pellet hits required to kill the game.

Figure 2: Pattern versus Range
For a given shot size and load, each 30 inch reference circle will contain approximately the same number of pellets

How Does a Choke Control the Spread of Shot Leaving the Muzzle?

The most common explanation of the functioning of the choke is that it works the same way as the nozzle of a hose, but this concept is totally incorrect. When you constrict the flow of liquid from a hose the liquid speeds up to get out of the nozzle - the smaller the hose nozzle the faster the stream of liquid coming from the hose and the higher the pressure in the hose itself.

But the shot charge is not a packet of liquid. It consists of a concentrated mass of individual shot pellets being pushed up to the muzzle by gas pressure at about 500 pounds per square inch. Each individual pellet is being squeezed tightly by every pellet in contact with it (hence all the little flats on the surface of the fired pellets) and the velocity of these pellets can only be increased by an increase in gas pressure within the barrel, which clearly cannot happen. In fact, the opposite occurs, the shot column is slowed down by the impact of the shot column with the taper of the choke.

But not all pellets are slowed down to the same extent. The layer of pellets at the front of the column are only being pushed by the pellets behind them, as those pellets are slowed by the taper of the choke walls the contact pressure on the front layer of pellets is reduced and they experience less disturbance as they exit the barrel resulting in less tendency to spread. Obviously, shot leaving a cylinder bored barrel is experiencing the full muzzle pressure of 500 pound per square inch, the other extreme, that corresponding to no muzzle pressure being experienced by the front row of pellets exiting the muzzle, corresponds to full choke. For the front row of pellets to be free from influence from those behind it then there must be either a space between the two layers of pellets or at least a "non touching" condition must exist. Once the "non touching" condition is produced by the choke nothing can be gained by further increasing the degree of choke as the layer of pellets exiting the muzzle are already in free flight.

If this is so, then why do modem cartridges give tighter patterns than the older paper cased cartridges with the overshot card wads? The explanation is simple. Firstly, the petals of the plastic shot cup prevent contact between the outer layer of shot and the bore of the gun, thus greatly reducing abrasion of the pellet's surface as it travels up the bore. Secondly, the base of the wad acts as a much better shock absorber than the old over powder felt wad which reduces the contact pressure between the pellets as they accelerate up the bore thus reducing surface damage. Lastly, the old over card wad often got caught up in the shot charge as it left the barrel; it had to be pushed out of the way by the shot charge as it lost its velocity much more quickly than the shot itself.

Is there any evidence to support this explanation as to how a choked barrel controls the spread of the shot leaving a barrel? Well yes, there is. Fig. 3 shows an illustration of spark photographs taken in the mid 1920's of shot charges photographed at 24 inches from the muzzle of the gun.

Figure 3: Shape of Shot Charge 24 Inches From the Muzzle

The first illustration is the charge fired from a cylinder bore barrel; the second, from a full choked barrel. It is also a fact that the shot velocity from a cylinder bored barrel is typically 50 fps, more or less, than that measured from the same barrel fitted with a full choke. So a velocity loss of about 50 fps is sufficient to free the leading row of shot exiting the muzzle from the pressure normally exerted on it by the remaining layers of shot still within the influence of the choke.

Now, if a shot column is suddenly checked by an obstruction as it travels down the bore a "ring bulge" is normally formed in the barrel at the base of the wad. So why is it that a choke can check the velocity of the shot charge without causing a ring bulge in the barrel? The answer is that a ring bulge is caused by the propellant gases piling up on the base of the shot charge as its velocity is checked; the resulting local pressure wave build up is reflected back to the breech, and then back again to the base of the shot column where it exerts a highly local radial pressure on the walls of the barrel, hence the typical shape of the ring bulge. But while the pressure wave was travelling from the base of the shot column back to the breech, and then forward again to the base of the shot column, the shot column has travelled further down the bore, so the ring bulge forms at a point ahead of the original site of impact.

For a light obstruction such as a choke the ring bulge typically forms 3/4 to 1 1/4 inch nearer the muzzle than the original position of the rear face of the obstruction. As a choke is always placed very close to the muzzle of the barrels the shot column has already exited the muzzle before the reflected pressure wave can impact on the base of the shot column. So no ring bulge.

Good Shooting!