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 Post subject: Shotgun shells, Chambers and Forcing Cones
PostPosted: Fri Jun 29, 2018 3:57 pm 
Field Grade

Joined: Wed Sep 05, 2012 7:31 am
Posts: 38
Location: New Bern, NC
A little straight talk on shot shells, shotgun chambers and forcing cones is needed to clear up some of the confusion and misinformation frequently posted on shotgun forums.

When talking about these things it is important to account for the era in which a shotgun was made as unfired and fired shell lengths and chamber length have varied through the years but have become more standardized in the last half century.

The fundamental anatomy of a shotgun barrel begins with the straight walled, cylindrical portion known as the chamber, in which a shot shell resides during firing. That is the portion denoted on the barrel and boxes of shot shells, as the chamber length and will be, 2 ¾”, 3” etc. Older shotguns may not have such notation. The chamber length is NOT the length of unfired shot shell but the length of a fired case that opens up into the straight wall chamber. When fired the casing opens up to release the shot and in doing so becomes longer, as opened, than it was closed. All shot shells are closed on the end by a crimp. Most all modern shells are closed with some type of star crimp but shells from older eras had simple rolled crimps with cardboard disc holding the shot in place. Rolled crimp shells open up shorter than star crimp shells of the same unfired length. The length of unfired case will vary a little among manufacturers by the type of crimping they use but thanks to the standardizing of “Sporting Arms and Ammunition Manufacturers Institute” (SAAMI) they will open up to their stated chamber length. Do not, on any gauge, be mislead because you can drop a 3” shot shell into the chamber of a 2 ¾” gun. An unfired 12 ga., 3” shell is only about 2 ¾” in length and will drop into the chamber of a 2 ¾” gun because the chamber is sized for an opened up shell case not a closed one. But, when fired a 3” shell opens up about another ¼”. Then there is no straight wall chamber length left in a 2 ¾” chamber to accept it and it has to open up into the smaller diameter of the forcing cone area of the barrel. That condition amounts to a constriction and leads to high barrel pressures that can be damaging to the gun and shooter.

Many older guns will have short chambers and were designed for a variety of chamber lengths as there were not always industry standards that now exist for modern shot shell and firearms manufacturers. Older guns can be re-chambered and their forcing cones lengthened to handle modern shells. Many shooters get away with it in older guns but shooting modern shells in an old gun chamber that was not designed for them is a bad practice. Many modern guns are designed to handle more than one shell length with some compromise in performance to handle different lengths.

Immediately following the straight wall cylindrical chamber there will be a conical section known as the forcing cone and there is no set length or angle of this section and it has varied significantly over time as shot shells have improved. The forcing cone will begin at the diameter or the chamber and reduce, over some short distance, to the bore diameter of the gauge shotgun it is. From there the barrel will be bore diameter to the muzzle where various size constrictions are applied known as choke. The intent of the forcing cone, before the near universal use of shot cups in shot shells was to force the mass of shot into a compact column coming down the barrel and minimize pressure gas leakage through and around the shot column. With modern shot cups the shot stays together inside the cup and the cup resists gas leakage until exiting the barrel. With shot cups the forcing cone doesn’t play its original role. However, one can still buy shot shells that don’t have shot cups and the forcing cone is still important for those shells.




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 Post subject: Re: Shotgun shells, Chambers and Forcing Cones
PostPosted: Sat Jun 30, 2018 6:32 am 
Crown Grade

Joined: Sat Feb 05, 2005 4:51 pm
Posts: 10961
Location: Phoenixville PA
re: forcing cones:

In the days of thicker-walled paper hulls, when bore diameters were pretty-much the same as the ID of the shotshell, and paper or felt wads were used, a very short forcing cone was required to prevent gas leakage into the shot column and causing lousy patterns. When thin-walled plastic shells and obturating plastic wads came on the scene (approx. 1960s), that all changed, and today's more gradual forcing cones came on the scene and have a definite advantage with plastics. BUT ..... I have observed no proven advantage to "extra-long" forcing cones, despite manufacturer's claims.

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 Post subject: Re: Shotgun shells, Chambers and Forcing Cones
PostPosted: Sun Jul 01, 2018 9:17 am 
Diamond Grade
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Joined: Thu Jan 27, 2011 3:35 pm
Posts: 1919
Location: AZ (heart in KS)
"Older guns can be re-chambered and their forcing cones lengthened to handle modern shells."

Can...doesn't mean should.

Pics help, courtesy of Hallowell & Co.

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The the external taper of the usual thick walled U.S. maker's double is more gradual than the angle of the forcing cone, so the wall thickness may actually be greater than the end of the chamber wall thickness. This may not apply to small gauge or light weight British 12 bores.
Cones can be lengthened EXPERTLY while maintaining adequate wall thickness, and doing so does not take a British gun out of proof. Lengthening the chamber, esp. 2 1/2" to 2 3/4" WILL lessen the wall thickness at the end of the chamber, NOT a good idea when shotshell pressures are highest 1" - 3" from the breech.

Done wrong you might get this; a bulge

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or worse

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 Post subject: Re: Shotgun shells, Chambers and Forcing Cones
PostPosted: Mon Jul 02, 2018 6:16 am 
Crown Grade

Joined: Sat Feb 05, 2005 4:51 pm
Posts: 10961
Location: Phoenixville PA
Drew is right, of course. And I have never had forcing cones lengthened on any of my old guns, and don't plan to, as they break clays and kill birds and bunnies just fine, thank you!

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 Post subject: Re: Shotgun shells, Chambers and Forcing Cones
PostPosted: Mon Jul 02, 2018 10:21 am 
Field Grade

Joined: Tue Apr 24, 2018 11:14 am
Posts: 51
Thank you for the lesson, very useful information.




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