doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 2.5 inch loads Classic Shooting Company (pics)

Just recieved a shipment of 2 1/2 inch 20ga loads from them, I opened the box and noticed the shells are marked 67mm, I thought 2 1/2 was 65mm? I compared them to some Gamebore loads and they are longer, are they safe to shoot in 2 1/2 chambered 20's?

yes

What, EXACTLY, is the chamber length of your 20g gun.

British 2 1/2" chambers are (usually) just that; 63.5 mm
European 65 mm chambers are 2.56"
European 67 mm chambers are 2.64"
U.S. 16g 2 9/16" chambers are 65.1 mm
12g 2 5/8" chambers are 66.675 mm
12g 2 3/4" chambers are 69.85 mm
Some just after the turn-of-the-century U.S. maker 20g chambers were cut slightly shorter than 2 1/2".
.....2 3/8" = 2.375 = 60mm; 2.4" = 61 mm; 2.45� = 62.23 mm.

Short non-boring answer: Sure, if the 65mm load is "safe" in your gun, the 67mm will be.

Its a Stevens 345 with 2 1/2 inch chambers, I'll get the chambers measured

RA, expect you'll be OK in a Stevens. Most of the time, those 67MM shells do work OK in 2 1/2" guns. The exception seems to be if you've got an old gun with short, sharply tapered forcing cones. I think it was Charles Fergus who discussed this in a Shooting Sportsman article from several years ago. He had a couple British 12's, one from the 1930's and the other, I think, from the 1890's. And he had a bunch of those 67MM shells--which say right on the box (if they're British) that they're OK in guns with 2 1/2" chambers. They were fine in his more modern gun, but he was blowing the ends off them in his older gun. Appears that the forcing cones were the culprit. He switched to true 2 1/2" shells in the older gun and had no problems.

What Larry said.
Left - 67mm Eley 28gm 12g; Right - CompX 65mm 21 gm.

As I recall the Ferguson story in SSM the ends of his chambers were essentially a step rather than a cone. This was allowing the ends of the fired shells to actually unfold into the bore itself. SAAMI Specs on 12 gauge chambers for 2 3/4" shells for many years specified a minimum length of 2.6136" with a maximum of 5� (10� included) taper from a minimum diameter of .798". Max fired case was listed @ 2.760". A max length hull in a minimum chamber would thuis lap into the cone by a length of .1464. Cone diameter at this point would thus be .772" or .026" smaller than the chamber end. This amount can well be tolerated. The end of the hull lapping into the bore proper is a "VERY BAD" idea as Mr Ferguson found out. He was fortunate indeed that nothing more occurred than cutting off the ends of the hulls, it could well have been a lot more dramatical.
Moral, on an old gun always check out the chamber to see exactly what it does have. A simple look down the bores from the breech would have revealed that sharp step as opposed to a tapered cone, giving forewarning not to use a hull which was longer than the chamber..
I don't recall the particulars on make & age of Mr Ferguson's guns, but I would be Extremely surprised if one as late as the 1930's had this type of chamber. Not many even from the 1890's would have. This is more something one would expect to possibly encounter in a "Very Early" breech loader.

In the UK where 2 1/2" nominal chambers are still quite common, the following 'rules of thumb' are applied for 2 1/2" marked chambers;
65mm is fine.
67mm is usually fine - the exception being strictly 2 1/2" chambers with a sharp forcing cone.
70mm is NOT suitable.

Most 2 1/2" proofed English guns I have measured are typically about 2 5/8" with a fairly spread cone.

All the UK (CIP) marked cartridge boxes I have seen here for 67mm (or 67.5) all state suitable for 2 1/2" chambered guns.

Awesome thanks guys, I did get a chance to fire a few rounds, the rolled crimp appeared to be more pinched at the ends when ejected than the straight 2.5 gamebore shells which appeared to be more open after ejection, I'll get some pics up of the two shortly for visual comparison.

2�" =2.500" = 63.5mm
2 9/16" = 2.2.5625" = 65.09mm
2 5/8" =2.625" = 66.675mm
2 3/4" =2.750" = 69.85mm
2 7/8" = 2.875" =73.025mm
3" = 3.00" = 76.2mm

As long as the chamber is long enough the crimp is not jammed into the cone & the cone is long enough the diameter at end of hu8ll is not restricted by about .75m (.030") below end of chamber diameter the the important thing is not the actual length of the hull BUT the Load therein.
Speaking here primarily of British proofed guns those proofed for 2 3/4" are proofed heavier than those proofed for 2�" loads. The heavier loads should not be fired in the lighter proofed guns regardless of what length of hull you manage to put it in. Far, Far better to fire a 2�" load in a 2 3/4" hull than to fire a 2 3/4" load in a 2�" hull.