doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 12 Gauge Shell Chamber Dimensions

Need your help: it appears the breach end of my newly acquired
Fox SXS barrels have been machined but the depth of the shell flange area has not been bored deeper. This is causing the off face to go from less than 0.0015 inch with an empty chamber to 0.013 inch when a Winchester Bismuth 12 gauge shell is put in the chamber. The measurements are taken with the breach firmly closed.

My "virgin" A grade Fox has the following dimensions:

* Depth of straight walled portion 0.044 inch
* Depth of straight and beveled portions 0.069 inch

Note: This gun has never had any problems with changing
off face dimensions regardless of the brand of shells
used.
Also, It has never miss fired--40 years of use.

The "altered" gun's dimensions:

* Depth of straight walled portion 0.032 inch
* Depth of straight and beveled portions 0.050 inch

I would appreciate knowing the correct dimensions and a
gunsmith in the Northwest who can do this work. Any estimate
as to the cost to do this work?

Thanks

Fox SXS

FSS,
The way a shotgun chamber rim depth is dimensioned is not condusive to using and simple measuring tool like a caliper to measure. The rim depth is dimensioned at a "gauge diameter" approximately in the middle of the 55 degree angled rim cut. That is why headspace gauges are used. They are available from Brownells and I recommend that anyone with a lot of guns add them to their basic tool collection. They are used in a set. The "Go" gauge should go in and not protrude from the barrel breachface anywhere. The "No-Go" should protrude from the breachface in at least some areas. Keep in mind that a sxs breachface is not perpendicular to the chamber axis in most cases. The breach face will likely be angled slightly top to bottom.

I used to have the 12g chamber dimension drawing on my computer, but I can't seem to find it. I acquired it from Clymer Tool who makes chamber reamers and gauges. If you buy a set of chamber headspace gauges directly from Clymer, he will send you the drawing free of charge if you ask.

I once owned a factory original Sterlingworth that had shallow rim cuts in its extractor. The gun acted just like yours, tight closing with shells in the chambers and forcing the barrels off face. I had a piloted countersink made up at a tooling shop and deepened the rim cuts by hand. PS, I don't have it any more, loaned it out and never got it back, long story.

I don't know if this vignette will help you because you say you have new barrels. I didn't catch if they were extractor or ejector. If you're using the extractor or ejectors from a different set of barrels their rim cuts may not align with the cuts already in the barrels, same scenario as with the SW I wrote about here. If the new barrels came with the extractor or ejectors I would suspect the rim cut depth before I would a tight chamber. Just my own thought process.... hope this helps.

If you are handy with tools you might check Brownells shotgun chambering stuff to see if they have rim cutters and/or chambering reamers with integral rim cut. I haven't dealt with Clymer so I don't know what they have. Sorry, I don't have the name of a good gunsmith in the NW. Silvers

Keith Karcher is in the NW and should be able to help you. He has done some very nice barrel work for me and rim depth should be NO challenge for him.
bill

I have chamber dimensions taken from Roy Dunlaps book "Gunsmithing". While several years old I believe these are still standard. Minimum dimensions for the 12ga are as follows; dia of outer rim 0.887", then 55° angle as Chuck stated. Note this angle is measured up from the horizontal, thus an included angle of 110°. The gauge point for depth is .0546 to a point on the angle @ 0.850" dia. An 0.020" radius is called for at the juncture of the angle with chamber body, so as Chuck pointed out there is no positive point to measure here.
While the No-Go gauges are handy for one doing a "lot" of bbl work, personally I never bothered to buy any. When I had need for gauging I would simply buy the "GO" gauge & cut varying thicknesses of shim stock to check how much clearance was there. As Chuck mentioned the breech of a dbl is not normally square to bore axis so if using this method the shim should be full gauge dia. Also note "NO" force should be applied when gauging. The gun should close easily on the Go gauge & not close on the No-Go, though most any in-tolerance gun "Can BE" closed on a no-go gauge if force is applied.

If you have a gunsmith mess with them, decide in advance whether or not you want the chambers cut to 2 3/4" if they are now 2 9/16". The barrels you have may be 2 3/4" chambered already, I dunno. If they are short chambers, some people want to keep the short chambers. Most gunsmiths will simply stick a full chamber reamer in there and cut not only the rim deeper, but the chamber longer as well. Make up your mind ahead of doing this so you're fully educated on what you want him to do.

Chuck, good catch - specifying chamber length beforehand if FOX SxS wants to go the gunsmith route or buy a combo chamber and rim reamer himself. That's why I had the rim countersink made up, wanted to stay with the original 2-5/8" chambers and just cut the rim recesses deeper. Silvers

I wish to thank Chuck H.,Silvers, bill schodlatz and 2-piper
for the valuable feedback. You have given me good data and
some food for thought.

Thanks

FOX SXS