Why should you consider special custom choke work and custom choke tubes?
Many factory choke tube holes and some aftermarket-type installations are not in line with the bore of the shotgun barrel: in other words, crooked. A choke hole that is misaligned is more likely to not shoot where you expect, and the choke tube that you use will have the performance affected by that off-line condition.
The tubes provided for the factory and aftermarket chokes being installed are ALL made with a lack of precise fit to reduce the possibility of danger from the prevalent off-center condition, and in the aftermarket-type case, to also allow the choke to fit a certain range of bore sizes. In any case, a lack of match between the expected bore size and the manufactured choke size will create an uncertain amount of actual choke, especially if the apparent bore size is unknown. The exit size of a choke at .700” will create different results when used in barrels of .720” vs. another at .733”, in other words, a “modified” choke and a “full” choke 12 gauge, but the same tube size is used in both situations. The tube would have more internal “jump” when in the smaller bore barrel and have less total length of the tube providing any contact and choking action. Some extreme examples can have 40% or more of the tube length be jumped before the shot makes appreciable contact, and that is when the tube is in an aligned hole. The factory chokes may be used in a precisely installed hole, but the tube fit of a custom choke will be lacking.
Exceptions to this situation are possible when a factory tube intended for a standard bore is then installed into a backbored barrel, where the bore diameter may be more closely matched to the tube entry end. Occasionally an aftermarket tube installation will be done to a barrel with a bore close to the maximum allowable size for safe clearance. Both situations, when installed with sufficient alignment to avoid having the tube edge intrude into the bore, will be closer to the ideal custom choke installation, but still will be limited by standardized length and choke exit dimensions, and will typically be of excess constriction compared to typical markings meant to correlate to a standard bore size. A choke marked “improved cylinder” can actually be a full choke, as it was in one particular example done here.
A precision custom choke installation and individually-fit choke tubes have none of the limitations that are present in mass produced choke holes and tubes. The pattern will be more likely to be in line with the point of aim when the tube hole is in alignment with the shotgun bore, and the patterns will likely be more uniform when the shot is constricted evenly through the entire choking action. An off-center choke will have the shot strike first on one side of the choke and have an uneven action. If the tube is held tilted as well, the shot will be forced to change the direction of travel during the choking action. Pellet deformity will be heavier on the shot column side exposed to the angled interference and cannot avoid having a detrimental effect on the pattern uniformity of any shotgun load. Such a tilt may also contribute to different points of impact between open and tight choke tubes.
An open choke may only strike against one side of the shot column to any significant degree. The more complete contact and choking of a tighter choke may guide the shot column more to the off-center side than the open choke.
Now that the limitations of mass production chokes and improperly aligned barrel threads has been explained, you can choose to not be limited by the chokes installed in your factory barrel or typically provided from aftermarket installers. Only when you have all of the facts can your decision truly be an informed decision.
The problem of crooked factory choke tubes should be considered whenever purchasing any barrel with tubes already installed. Ideally, if you prefer a particular length, and the barrel has a crooked choke that needs to be replaced, buy the barrel at a length longer than what you want so the barrel can be cut to the preferred length and have a precision hole installed. If that choice is not practical due to circumstances, remember this mantra: a straight choke in a shorter barrel has got to be better than a longer barrel with a crooked choke. Special choke installations can have longer extended choke tubes that eliminate the difference in “feel” of swing by replacing the lost length and mass of the removed barrel section. The main loss is of the rib length, but the tube will never be seen while looking down the barrel.
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Shotgun barrel improvement
Shotgun barrels have had a static internal design for most of the time that self-contained cartridges have been in use. Only in recent history have the manufacturers started to incorporate changes that used to be considered as “custom”. The factories, for the longest time, had looked askance at any modifications as a taboo subject with a high disapproval rating.
Factories had always considered backboring to offer negligible difference, at least until they decided to offer a model with a new backbored barrel for the target shooters. Suddenly, backboring went from useless to fantastic, and now many lines have models with oversized bores.
Winchester was the first manufacturer to have any substantial production of factory installed choke tubes and had a record for the most models available with interchangeable chokes for a rather long time. The other factories probably hoped that it was just a fad, but now nearly all current models have choke-tubed barrels. The last refuge seems to be some of the classic-style side-by-side doubles keeping the fixed choke tradition alive a while longer.
The latest advancement that has been “discovered” by the manufacturers is the lengthening of the chamber’s forcing cone. The main difference between factories is the amount of difference from the classic configuration to the new and improved length. Some are twice as long as before, and some extend to several times longer. Those longer version forcing cones are closer to the optimal length that I have been providing for over 20 years, but the factories don’t have the same finish or blending that has allowed my versions to offer the greatest benefit. The whole point of lengthening the forcing cone is to improve the pattern efficiency for more even pellet distribution, flyer reduction, and in my version’s case, apparent increase in the average muzzle velocity (chronographed by me during initial experiments).
See the drawing image of the chamber and related components. Notice that the end of the shell chamber area (which tapers slightly) has a funnel zone that squeezes the shot/wad grouping to fit into the shotgun bore. This funnel taper is altered for longer forcing cones, so the squeezing action has a longer time and distance to accomplish the same amount of shrinkage. Minor length changes produce subtle differences. There needs to be substantial length change to produce obvious performance change, but continued improvement (reducing lead pellet deformities, for example) has diminishing returns from additional length.
Backboring example amounts used in the image are typical, with .012” or less difference being common. Chamber to bore difference (.798 to .728 nominal, .070” total) being altered by .012” is a 17% reduction in diameter difference, which calculates into a 3.2% reduction in area (sq. in.) difference. Not exactly a revolutionary concept, when seen in that light.
Chamber length that is longer than the shell forces the wad/shot to enlarge to the size of the chamber after it passes the end of the hull. Then the shot charge gains speed before finally making contact with the forcing cone (of whatever length). There is the physics concept that proves that the potential of a shorter shell in a longer chamber is never as good as when the shorter shell has a chamber length that is a closer match.
Lead pellet deformity also occurs from firing acceleration forces, so no pellet will leave the barrel as round as it went into the shell. Pellets transiting a long forcing cone have reduced flat spotting, but these pellets can be deformed when squeezing through the choke. A short length choke with heavy constriction has deforming effects that can negate much (or all) of the benefit gained at the chamber end.
The longer forcing cones that I have done have had many hunters recounting how much farther their barrels can reach, even when they have fairly open chokes. Pellets that are smoother will fly straighter, lose less velocity than damaged pellets, and consequently reach and be effective to a longer range. Open chokes are not going to pattern more tightly at close range, but will have less spreading as the range increases. That is why open chokes (even in smaller gauges) have demonstrated remarkable improvements from the special longer forcing cones.
Chokes with heavy amounts of constriction should not be as short as many of the factory-style choke tubes, but factories have tended to be slow to change, as we have seen. Lately there has been the introduction of longer versions of choke tubes in some models, and that can result in better quality pellets flying downrange.
I do thank the factories for getting on the bandwagon of shotgun improvement. I have been educating shooters in the concepts for optimal performance to suit their situation(s) for over 3 decades.