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.
Last edited by JohnfromUK; 11/20/16 03:33 PM.
