HI, this has nothing to do with headspace. There are just so many posts, I picked the first one I came to. If you don't know the answer, maybe you know a member better suited for this question. TYHe rifle my Customer brought inot my shop has the number 5849 on it Slide cover. I was told that the number is under a certain number that I can't recalll that distinguishes it as a model BEFORE the barrel change, which woould make it a 7.5mm. Ii I am correct. Aftfer the arrel change in the 60's it went to a 7.62x51mm. Do you know the correct answer or a member who might? Thank youI have some pictures but, they aren't as clear as I would like. I am goung to try and hilight the emblems on the receiver and barrel the next time it is in the shop just so they can be seen.