Headspace on an M1917

[Patrick Chadwick]

Dear Edward,
I was delighted to see your photos of the 308 cartridge fired in a 30-06 chamber. But your comment "One half inch of excess headspace ..." seems to be a trifle misleading.

From your contributions, I deduce that you have the CIP or SAAMI data to check this out: The shoulder diameter of the 308 case is actually slightly larger than the diameter of the 30-06 case at the same distance from the base. So if you feed a 308 case into a 30-06 chamber it will jam. I have mentioned this before, in describing how a clueless person next to me at the range was cursing because it was so difficult to close the bolt. There will,in this situation, be actually zero head clearance on the 308 case.

A genuine 1/2" of clearance would make it impossible to fire a cartridge - the firing pin would not be long enough!
The jamming/camming effect will obviously be highly sensitive to chamber size and degree of resizing applied to the case. I suggest you try it out.

Patrick