Good points, Miller--and from what Fergus wrote, I think the situation he encountered was as you describe it.

I don't have a recent Brit 2 1/2" gun to measure. Most, of course, are now chambered 2 3/4"/70mm. The older 2 1/2" guns reproofed recently are remarked, I think, 65mm--even though I do not believe any actual modification in length has been made. So even though 2 1/2" does not equal 65mm, I think the Brits treat the two as being the same. I found it interesting that Fergus' old gun passed reproof some time after 1954. Apparently the proof loads didn't produce anything unusual, even though the 67mm shells did.

I think a reasonable conclusion is this: mouth of the unfired hull extending into the bore itself, or into a very short and sharp forcing cone, may very well produce dangerous results in terms of a pressure spike, visible damage to the hull, and perhaps increased recoil.