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Rifling of a 105 mm Royal Ordnance L7 tank gun Conventional rifling of a 90 mm M75 cannon (production year 1891, Austria-Hungary) Rifling in a GAU-8 autocannon. Rifling is the term for helical grooves machined into the internal surface of a firearms's barrel for imparting a spin to a projectile to improve its aerodynamic stability and accuracy.
The original 6.5×52mm barrel design, developed by the Brescia Arsenal at the same time as the cartridge before development of the M91 Carcano rifle itself, used a gain twist barrel with deep rifling to reduce wear and give consistent accuracy.
This revolver uses gain-twist rifling, starting with a slow twist rate of one turn in 100 in and gradually increasing up to one turn in 20 in to accommodate the combination of a heavy bullet and high pressure and velocity typical for the round. [3]
Thus the optimum rate of twist for this bullet should be approximately 12 inches per turn. The typical twist of .30-06 caliber rifle barrels is 10 inches per turn, accommodating heavier bullets than in this example. A different twist rate often helps explain why some bullets work better in certain rifles when fired under similar conditions.
Another issue that has an effect on the barrel's hold on the bullet is the rifling. When the bullet is fired, it is forced into the rifling, which cuts or "engraves" the surface of the bullet. If the rifling is a constant twist, then the rifling rides in the grooves engraved in the bullet, and everything is secure and sealed.
Some airgun rifles have a short cone towards the muzzle to improve the speed of the bullet. [clarification needed] A slight gain in the rifling twist, or a slight tapering down of the bore, will still ensure the tight fit of bullet to bore, so when selecting a barrel, the tighter or faster twist end should be chosen for the muzzle. [37] [38] [39]
The barrel length is reduced to 310 mm. The 8 inch twist rate of the barrel is especially adapted to stabilize the long heavy projectiles used in subsonic ammunition. The propellant charge of subsonic ammunition is reduced to keep the bullet velocity (300–320 m/s (980–1,050 ft/s) below the speed of sound (~ 340 m/s (1,100 ft/s)).
To make the 6.5×68mm an awesome long-range cartridge, rifle barrels with a 200 mm to 230 mm (1 in 7.9 to 9 inch) twist rate are a better choice. Due to the large case capacity in relation to the 6.5 mm (.264 inch) caliber bore size the 6.5×68mm is very harsh on barrels. The 6.5×68mm typically wears out a rifle barrel in 500 to 1000 rounds.