If you are not loading a belted case, you need not read any further. 
This note is an observance I have made and I thought it might be good to pass it along.
In some belted Magnum cases, the inside cavity of the case extends down past the forward
edge of the belt. If you have such cases, measuring case head expansion may be deceptive.
Here is why: The forward edge of the belt may expand considerably more than the actual
solid "head" of the case, giving the idea that a lot more expansion occurred than actually did.
The measurement must be taken at the rear of the belt only. This requires a blade micrometer or even
a very accurate Vernier caliper with the thin blades at the tip of the caliper jaws. If you use a conventional
Micrometer across the whole belt, you will get readings from .0025" to .003" larger than the case head
actually expanded.
I have personal examples: I have some newly manufactured 308 Norma Magnum cases [Norma headstamp]
Unfired, measured across the whole belt, they are .5285" in diameter.
After firing with a known safe load, if you measure across the whole belt, you will get .5310" - .5315"
This would apparently give expansion of .0025" to .003", an excessive amount of expansion.
However, using a blade micrometer, and measuring across the back of the belt only, the measurement is
.5285" - .5290" .....perfectly acceptable expansion for the first firing.
I ran into the same issue with my 257 Weatherby Magnum when developing loads for it.
Just thought some might find this information interesting. Dave.
This note is an observance I have made and I thought it might be good to pass it along.
In some belted Magnum cases, the inside cavity of the case extends down past the forward
edge of the belt. If you have such cases, measuring case head expansion may be deceptive.
Here is why: The forward edge of the belt may expand considerably more than the actual
solid "head" of the case, giving the idea that a lot more expansion occurred than actually did.
The measurement must be taken at the rear of the belt only. This requires a blade micrometer or even
a very accurate Vernier caliper with the thin blades at the tip of the caliper jaws. If you use a conventional
Micrometer across the whole belt, you will get readings from .0025" to .003" larger than the case head
actually expanded.
I have personal examples: I have some newly manufactured 308 Norma Magnum cases [Norma headstamp]
Unfired, measured across the whole belt, they are .5285" in diameter.
After firing with a known safe load, if you measure across the whole belt, you will get .5310" - .5315"
This would apparently give expansion of .0025" to .003", an excessive amount of expansion.
However, using a blade micrometer, and measuring across the back of the belt only, the measurement is
.5285" - .5290" .....perfectly acceptable expansion for the first firing.
I ran into the same issue with my 257 Weatherby Magnum when developing loads for it.
Just thought some might find this information interesting. Dave.
