Brian in thread 11 makes a good point about eroded leeds. The gauge he mentions was used to test or measure the wear at the leed but seemed to fall into dis-use for rifles during the 60's or so but remained in use for Brens until the end (the Vickers had its own.....) But some used to swear by this gauge over all others as with it, you would see the wear for gas to pass the projectile and cause the projectile to become unstable even before it really started its way up the bore. Even more unstable with the boat tailed Mk8 stuff.

The moral here is that everyone should keep a small stock of standard military Mk7 ammo just for test/calibration purposes. Like finding a No32 telescope. Get some now while you can still get it resonably cheap and available.