The Shotgun Shop

(http://theshotgunshop.net)

 Gunsmithing

P.O. Box 212, Arnold MO 63010
  
636-282-4379   theshotgunshop@hotmail.com

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Mechanical refurbishment and custom choke work

Super-long forcing cones and chokes, all gauges

Custom choke installations in thin barrels 

Specialty Steel Shot and turkey chokes

Action smoothing for all firearms

Graduate of the Colorado School of Trades

20+ years experience

Specialist work available for: 

Classic American and Import Doubles  

Winchester Classics, including 

Model-12, 97, 37, plus parts

Remington SP-10/Ithaca Mag-10
 
 
 
 
 

The Shotgun Shop, P.O. Box 212, Arnold MO 63010 (636)-282-4379

 theshotgunshop@hotmail.com

 Shotgun Barrel Improvement, Part 1: Factory Style

 

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 2 decades.

 

Keep an eye on this space, more may be added.