The CIP, proof authority for members states that include most major shotgun manufacturing countries, state clearly that 67mm shells may be fired in 65mm chambers. But specifically warn against using other long shells in short chambers. Sixty seven millimeters is 2 5/8 inches according to my calculations.
We should be cautious with opinions on this subject considering that some not so knowledgeable readers read these posts. The safe practice, according to CIP is not to use 70mm (2 3/4) in shorter chambers. Same goes for 3 inch and so on.
That warning concerns factory shells, not reloads. And it's a reasonable warning where factory loads are concerned, because the SAAMI service pressure limit for 2 3/4" (or 3") 12ga is 11,500 psi, while the CIP service pressure limit for a "Standard Proof" gun--and all 2 1/2" guns are standard proof--is 10,730 psi.
If you compare the fired length of 67/67.5mm hulls to 2 3/4" hulls--I used to have quite a collection--you will find that, in some cases, the 67's are as long or slightly longer than nominal 2 3/4" hulls. Reloading 2 3/4" hulls to appropriate low pressures and shooting them in guns with 2 1/2" chambers isn't a dangerous practice--other than in the case of guns with very short and sharply-tapered forcing cones, per the above reference. Those would almost all be 19th century guns. And in the case of those guns, even 67mm shells should be avoided--in spite of CIP's approval of those shells for 2 1/2" guns.
