doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Chamber/Forcing Cone question

I have a really nice S x S 12 GA by Heym, made in the 30s, as original with 2 1/2" chambers, according to my chamber gauge. Forcing cones are about 3/4". I only shoot low pressure loads in my 12s (less than 6k, 3/4 oz).But they are 2 3/4". Is that safe and unlikely to damage the barrels? Barrels marked "Grade 2E Krupp Fluid Steel Barrels". Thank you! Gil

Gil,
You are fine, as is. This has been discussed, ad nauseam, here, and elsewhere. 2 3/4” low pressure reloads, in 2 1/2” chambers, is a non issue.

Best,
Ted

Bell's 12 gauge study showed an average increase in pressure of 700 psi using 2 3/4" shells in 2 1/2" chambers. The increase in one load was 1200 psi.
He also demonstrated the lengthening forcing cones, without lengthening the chamber, does lessen that pressure increase somewhat.

It would be prudent (and easy) to check for 'feathering' of the case mouth in spent shells, and if so the mouth has entered the forcing cone.
67 mm Left; 65 mm on Right



It has also been observed that nominally 2 3/4" shells after firing are often shorter



And, again, WE HAVE NO DATA FOR USING LONG FOR CHAMBER 16G AND 20G SHELLS

Seems to be variable lengths of hull incorporated in the crimp formations in the above image. So presumably there will be varied degree of protrusion into chamber cone on firing the different shells - not counting the amount of stretch by different plastics....and hull wall thickness.....also diameter of wadding.

I concur with Ted and Drew in the opinion your ok with doing as you described.

Though I add you could cut your hulls to 2.5 inches, put the same low pressure 2 3/4 data into them, then roll crimp.

One of many good reason to reload. I trimmed 5,000 hulls to 67MM for all 2 1/2" chambered gun needs. 67MM is considered proper length hulls for 2 1/2" or 65MM chambered guns. And I only use one type of hull for all my short shell loads. Every shell is loaded to the same pressure, same velocity and only vary in shot size. If I can not do it with 1 ounce of 7, 7 1/2, 8 or 9's I need to reach for a bigger gun or payload. Even my 3" chambered British guns seem to do perfectly fine with 67Mm, 1 ounce loads. I do shoot 2 3/4" loads in them if needed, but they are proofed for 4 ton not the lesser 2 1/2" proof pressure levels. Using factor shells is much more difficult and more expensive when you have short chambered guns. Hence one of my reasons to reload. Plus I enjoy doing it.

Burrard shows tables of data which compare different loads fired in proof testing barrels with and without a chamber cone. May be of interest to those who are in same position as the OP.

I couldn't find that in Vol. 3 of Burrard's "The Modern Shotgun"
Would you please share his data?

More discussion of chamberless barrels is here
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=621506

I am not opposed to roll crimping, but if you are using a MEC, either single stage or progressive and don't want to alter the press or set up a separate station to roll crimp, you can simply insert an overshot card before crimping. I use a 20 ga over shot card in 12 ga shells. This was detailed in Shooting Sportsmen many years ago.

I have a French 8 gauge shotgun that was made about 1878 with no forcing cones. I have seen a few but not many.