[SL1]

Probably the easiest way to measure the inside of a case neck with what most people have on hand is to measure the OD of the case neck with a caliper or even a micrometer, then measure the case wall thickness with a caliper and subtract twice the case neck thickness from the OD to get the ID.

BUT, when you do that, make sure that you take the measurements multiple times at the same place as well as multiple places on the same case neck. Multiple times at the same place allow you to see how much variability resutls from your tools and technique, while doing it at multiple places will allow you to see how much variation is really in the metal you are trying to measure.

You will probably find that the OD varies by a several thosandths, due to TWO causes: the inside opening is probably slightly oval, and the neck thickness is not uniform around the circumference (or even along the length) of the neck. So, this will give you some idea of the UNCERTAINTY of your measurements, as well as what is contributing the most to that uncertainty.

You could use pin gauges, but they will tell you only the diameter of the smallest circle that can fit into what may be an oval opening. The same goes for the inside diameter measurement devices that expand to fill the holes and then either have a direct readout or are "locked" and removed from the hole to be measured by calipers or micrometers.

For case necks, gettiing measurements of either thickness or diameter to finer than 0.001" should not add anything to your reloading capabilities.

SL1