There are some guns--not many, and most are older with very short and sharply angled forcing cones--that either won't work or won't work well with shells that have a fired length longer than the chamber. It's rare to find guns with true 65mm chambers that won't accept 67/67.5 mm shells, because that's the FIRED length. They're shorter than 65mm unfired. The more typical problem is, they'll fire but you'll end up with blown ends on the shells. Charles Fergus, for one, reported that problem in Shooting Sportsman. The slightly longer hulls worked fine in a between the wars Brit gun with short chambers, but he got blown ends and also stated a louder report and more recoil in a 19th century gun. Chambers the same length; forcing cone different. He reported that true 2 1/2" shells worked fine in the older Brit gun.
Had exactly the same situation here, with a Russian hunter to whom the late Geno told about Bell's experiments and that it's OK to use 70 mm shells in 65 mm chambers. The hunter experienced increase in recoil and blown heads; the shells were hand-loaded with roll crimp and were identical to the load which worked fine in 65 mm shells, except for the length and an extra fiber wad.
