doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Lengthened forcing cones and proof

If you lengthen a chamber by 1/4", you're removing a relatively large amount of metal in a relatively small area. If you lengthen a forcing cone by an inch or more, you're taking out less metal from a longer section of barrel. Lots of professionals will do both, but the ones that really know doubles are likely to hesitate more over punching chambers than they would over lengthening cones.

You are spot on there Larry. lengthening a chamber will always produce a thinner area in the barrel wall than what existed previously. As to whether or not that constitutes a hazard depends totally on what was there to begin with. Another factor certainly worthy of consideration is as to whether lengthening a chamber is going to encourage the use of heavier loaded shells.
Lengthening a cone is an altogether different matter. It very seldom produces a thinner area than what is already presently there.
I personally do not recommend either, but both can under some conditions be done with complete safety. A "Competent" gunsmith would evaluate the individual gun & "NOT" make a Blanket statement either For or Agin'it.

At the risk of repeating myself...I shall

In every U.S. maker vintage double that I have measured, a few with lengthened cones (of 1 1/2" or less), the wall thickness at the forcing cones was equal or greater to the wall thickness at the end of the chamber. The angle of the forcing cone is greater than that of the exterior taper of the barrel.

Apparent even in his low resolution image: Commission Internationale Permanente pour l'Epreuve des Armes � Feu Portatives (C.I.P) Shotgun Minimum Wall Thickness Recommendation



Please note "Standard Steel" is defined as tensile strength 700 - 849 N/sq. mm = 101,526-123,137 psi (modern 4140)
http://www.cip-bobp.org/sites/default/files/new_file/A-4-1_EN.pdf
10 & 12g end of chamber recommendation - .079"
20g end of chamber - .075"

Pressure-distance curve shows pressure has fallen significantly by 3" for BP, Bulk and Dense Smokeless, and DuPont Oval

For the barrel butcher fools, courtesy of Hallowell & Co.

RevDrDrew,

Looks obvious from the illustration that the real issue would be if the chambers are lengthened.

One thing I noticed is that the wall thickness minimum is less on the 20ga but 20ga usually run higher pressures. Any comments/ideas why? Something with hoop strength maybe?

Thanks Drew for posting these pics. As the old saying goes one picture is worth a Thousand words. Your pics so graphically illustrate what I was trying my best to say. If only the cone is lengthened (Not the chamber) in a quality manner it will be an extremely rare occasion where strength will be compromised. As to whether its good value for the costs incurred is for each to decide for themselves, personally I think not in most cases.

C.I.P. didn't ask my opinion Bill, but I don't think Federal Premium Wing-Shok High Velocity 20g loads are available in the EU

As you observed, C.I.P. 20g service and proof pressures are higher than 12
http://www.cip-bobp.org/homologation/en/tdcc_public?page=1&cartridge_type_id=7
12g 65mm and 70mm �standard proof� lead or steel (limited to no larger than 3.25 mm and max. fps 1,300) transducer psi
SERVICE pressure 740 BAR = 10,733 psi; PROOF 930 BAR = 13,489 psi
Both 65 and 70 mm 16g standard is SERVICE 780 BAR or 11,313 psi; PROOF 980 BAR or 14,214 psi.
Both 65 and 70 mm 20g standard is SERVICE 830 BAR or 12,038 psi; PROOF 1040 BAR or 15,084 psi.

And the Hunter Arms "Close Enough for Farm Implements" illustration

A couple of things of note here. First in this 1907 drawing Smith called for a bore diameter of .650" which is well under the nominal of .662". in later drawings this size was increased.
2nd the taper of the forcing cone figures to .150" per inch on the diamter. Based on the listed size of end of chamber & bore the 12 would have a cone of .460" length while the 16 would have one of .547", even though the angle is identical for each. SAAMI Specs have for sometime given a max angle here of 5�. This would give a taper to the cone's diameter of .175" per inch. With same specs as for above this would give a cone length in the 12 of .394" and in the 16 of 469".
I just checked the bore diameters on a 16ga Baker Black Beauty & A Lefever 16ga H grade. The Baker measured .663" with the Lefever going a bit oversize @ .670". Both have 28" barrels & both weigh in @ 6 3/4lbs. Neither are super lightweights but both handle & shoot well.

Interesting hunter arms drawing. No rim rad, just a chamfer.

A smaller diameter tube, of the same wall thickness as a larger diameter tube, will have less hoop stress at the same pressure.