doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com how to check chamber length

Patrick,
According to the markings on your gun, it was indeed a 2 1/2" 12 ga. with 1 1/8 oz. Birmingham nitro proofs. The bore diameter at the time of proof was .719" The right barrel has no choke and the left does.
Now you just need to check and see if the chambers were lengthened with one of the methods above. The small machinists slide rule works well and is cheap to acquire.

Blue Grouse,

Thanks. So, I understand the 1 1/8 is the proof load (I thought that marking might be the load). What tells you that the chamber was originally 2 1/2"? And that the bore diameter was .719"?

Thanks again for the help.

Patrick;
There is nothing on your gun that tells you it had "Exactly" .719" bores when proofed. The 13/1 tells you it would accept a .719" diameter gauge 9" down the bore, but would not accept a .729" (12ga) one. It could have thus been anywhere from .719" to .728". 1 1/8oz was the standard proof for the nominally chambered 2�" gun. The chamber actually likely measured from 2 9/16" to 2 5/8" (65-66.7mm) but the Brits called them 2�". This 1 1/8oz was the service charge, not the proof charge. A gun of this era carrying 2 3/4" (70mm) chambers was required to pass the heavier proof for 1�oz of shot. The former was essentially a 3-ton proof & the later a 3�-ton proof. These amount to approximately 8960psi & 9800psi as service loads.

The marks relate to the 1925 Rules of Proof. The 1 1/8th. ounce service charge denotes it has 2 1/2" chambers. They started putting chamber length on after 1954. I suggest you use only European ammunition that has the CIP mark on the box with 65mm. or 67.5mm. length cartridges. I know the 67.5 is longer than 2 1/2" but they are o.k. for that chamber length. The U.S. does not have comparible proof standards on either guns or cartridges and U.S. 2 3/4" stuff is generally a lot hotter as to pressure than European stuff. Please don't strain the old girl!

Midland guns are quite interesting in that a great many were as per the catalogue; although they did make one offs to order, if you can show a picture of the side of the action and let me know the type of fastening to the action and the forend I may be able to say what it is. They had very fanciful names for their guns. Sadly no records exsist. Lagopus.....

Gents, Thanks for the great help. I really enjoy the gun and have been shooting 2 3/4 shells through it. I'm thankful to have realized it may have 2 1/2 chambers and asked. I'll measure just to verify and then stop shooting it until I get back to the states and get correct ammo.

Lagopus, I'll put the info on the action and lock up in my other thread on this gun "Trying to date a Midland SxS"

Thanks again to all.

Patrick,
Bell showed in his studies published in the Doublegun Journal that there was only a slight pressure increase of a few hundred pounds shooting 2-3/4" shells in 2-1/2" chambers. As long as you're not shooting "magnum" loads, I wouldn't expect you'd have a problem.
Steve