Quote Originally Posted by Alan de Enfield View Post
Exactly my point - heresay easily becomes fact when the internet allows global interaction.

I can understand the theory that shorter (lighter) bullets will have less 'grip' or contact in the barrel and may have less accuracy but the 'escaping gas' and erosion theory eludes me - certainly when we look at the number of rounds we'll fire
Sure; one of the other throwaway line that supports a myth industry is "A few rounds of Mk7 destroys the accuracy of machine gun barrels." Without all of the supporting data, this is fairly meaningless.

What did they mean by "few": 3 or 4 rounds? 300 or 400, or 3,000 or 4,000? Viz, if a barrel has a normal service life of 10-15,000 rounds, then the proportional wear caused by a few tens of rounds is going to be of an order of magnitude so small that it'd be subsumed by other factors such as manufacturing tolerances and surface finishes in the new barrel, etc.

Without getting into the "accuracy" part - machine guns barrels aren't intended to be match barrels anyway, because they have to produce a "beaten zone" - there must be more to the MkVII/VIIz/VIIIz story than just the difference between cordite and nitro. E.g. I have a large quantity of MkVIIIz, and find that for some reason it shoots very well in P14s - whether they are very worn (presumably from a service life with cordite MkVII) or brand new bore condition. The performance in No4s varies from poor to exceptional with no obvious correlation to bore condition (again assuming that most bore wear in a well-used No4 has occurred with cordite ammo). Perhaps there are subtle variations in the form factor of the rifling itself that have an effect?