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Guys - I am sorry for any and all offence generated by my post. It is frustrating trying to get to the bottom of the 'problem', especially when we have seen no proof. We're told there are/were a number of guns thus damaged, with the lands of the rifling pressed out to the outside, even protruding or at lest visible, but none are brought foreward. Any movement of the lands must increase the bore diameter to a measurable degree - any measurements before and after?

BTW - Jacketed bullets or monometal bullets of groove diameter or slightly under as in the shank length of Barnes TSX's, do not "obturate" into the rifling. No modern bullet obturates as it engages the rifling. To obturate, bullets must be of ductile nature or material and they are undersize to the groove diameter in the first place, then expand to fill the rifling, as the minnie-bullets did which were shot in Civil War rifles and rifles around Europe at that time. That is obturation. "Obturation" is expansion in diameter which causes a reduction in length. Obturation is caused when the rearward pressure against the bullet exceeds it's elastic strength and causes it to foreshorten and expand into the rifling. This happens prior to the bullet's initial movement inside the bore. Simply put, the rear of the bullet moves before the front, causing it to expand in diameter but become slightly shorter.

Perhaps the word "Obturate" is being used because it 'sounds' knowledgeable. Unfortunately, it's use in this context of solid metal bullets is ridiculous.

I suspect you are trying to say the bullet is too hard and dense to allow the lands to engrave/impress into it, therefore causing the lands to press outwards. You are saying the bullet forces the lands outwards. If this was actually happening, there would be incredible pressure outwards from the bore of the barrel which would SHOW on a strain gauge as well as any other type of pressure measuring device. If there is no recording of excessive pressure, then physics says damage isn't being created.

Do you really have any idea how much breech PRESSURE, in PSI it would take to actually press the lands out through the metal of the barrel by a bullet. The bullet would have to be very hard indeed. Theree probably aren't any doubles rifles made, then or now, which could contain such pressures, let alone have this happen at normal pressures.
I don't believe you realise just exactly what you are claiming is happening.

Obturation is not only fine but necessary if you want to shoot undersized (under groove diameter) lead bullets, grooved lubricated or paper patched undersize bullets from your double rifle. The breech pressure must be high enough to cause obturation of the bullet metal being used, so the lead bullet will expand to engage the rifling. Obturation is necessary for shooting some bullets in some barrels, but generally doesn't happen in modern guns shooting modern bullets. It certainly won't happen in double rifles shooting jacketed bullets of sufficient integrity to withstand heavy animal structure, especially at the low pressures involved and necessary in these weaker actions.

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Daryl

"a gun without hammers is like a Spaniel without ears" King George V

Edited by Daryl_S (24/10/09 02:35 AM)

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