Accurate enough for the present purpose
Quote:
Originally Posted by
Hoss
How accurate does the measurement have to be? Unless I had calipers or a micrometer, I don't think I could get a very accurate reading this way, and if I had calipers, I wouldn't need to mess around with sticking a bullet into the muzzle.
True!
But measuring by inserting caliper jaws into a muzzle is very tricky, since
a) the curvature of the wall tends to produce a smaller reading, because the caliper jaws have a finite thickness, and
b) if the bore has an even number of grooves, you may measure groove or bore diameter, so you have to make more than one measurement at different positions, to see which is which, and
c) if the bore has an uneven number of grooves, you don't know where you are. So you can actually make a pretty good measurement of the diameter at the muzzle with the "twiddled bullet" method.
More importantly, if the muzzle is badly worn and has become bell-mouthed, then
d) calipers produce a falsely optimistic result, because of the jaws contacting further down the muzzle, where the wear is not so great.
Anyway, whichever method you use, it is accurate enough to let you identify the nominal bore size!
Quote:
Originally Posted by
Hoss
There are a lot of cartridges that are just a tiny bit off on the diameter but are completely different rounds. Heck there are plenty that actually have the same diameter and are still completely different rounds. I always figured the bores widened a little with use too. Maybe this will work because lugers were only made in a few specific calibers?
Exactly! - It's good enough for that! And, btw, bores are usually measured land-to-land, but the caliber designations are often inconsistent - e.g. .303 British has the bore(land) diameter, whereas .308 Win has the groove diameter. Metric calibers are usually (if I write "always", someone is bound to find an exception!) bore x case length - e.g 7.62x51 (7.62mm = .300")
Patrick
:wave: