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I had read that some combinations "lodge" on the body tapers - I read that 7.62 NATO lodges in a 30-06 chamber - headspace is totally irrelevant in that case - the case is "held" for the firing pin to strike that primer and set off the charge - some have said that the external extractor on various rifles (like Mauser 98) can do the same thing - hold the cartridge enough for the firing pin to strike the primer and set it off. Versus examples like 280 Rem - it will NOT chamber into a 30-06 chamber because the "headspace" dimension is different, but some will run a 30-06 case into a 280 Rem die and claim that to be their 280 Rem. In the past, I had a notion to re-chamber a 7x57 into 280 Rem - except 280 Rem chamber reamer will NOT clean up the very rear of the 7x57 chamber - even though the barrels (bores) can be the same internal size. I suspect that is what the OP has - someone has reamed an original 6.5x55 rifle to accept a larger 6.5 cartridge, and has NOT re-marked that barrel with the "new" chambering. Chances are good that bolt face is still properly sized for 6.5x55 which is a bit larger than 30-06, but likely "works".