Barrel Twist
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Barrel Twist


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Old 09-10-2011, 04:36 PM   #1
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Default Barrel Twist

After reading up on different rifles and their respective twists, I have come to a conclusion that the twist is nothing more than the rifling in the bore of the weapon. I understand that every weapon will have a twist for it's specific chambered round but what is the science behind it? Basically, what does the twist have to do with the acceleration of rounds that are extremely close in size?


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Old 09-10-2011, 04:53 PM   #2
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A good discussion

http://www.firearmstalk.com/forums/f20/understanding-barrel-twist-bullet-weights-44557/



Last edited by Gatoragn; 09-11-2011 at 12:18 PM.
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Old 09-10-2011, 07:47 PM   #3
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Thanks, it was very informative.
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Old 09-10-2011, 08:26 PM   #4
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Couple of bits of "twist" trivia that Sniper did not cover in that very good post-

First, the term "gain twist". Rarely encountered now, it was used in early Colt black powder revolvers, and in the Italian Carcano rifle. The rifling starts at the breech end almost straight, and the rate of twist increases as you move towards the muzzle. Allows the gun to "spin up" a bullet as it accelerates. The Carcano used an unusually long bullet that would tend to "strip" thru standard twist rifling.

Second, the term "Paradox". These were incredibly large black powder cartridge guns that wee known as 8 bore or 10 bore guns (think 8 guage or 10 gauge) Yes, they were freaking cannon- intended to knock an elephant down. At the breech end, the barrel was a smoothbore. Partway down the barrel, it became rifled. That is why they were called the Paradox.
paradox-muzzles.jpg
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Old 09-11-2011, 10:24 AM   #5
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Quote:
Originally Posted by Gatoragn View Post
A good discussion

http://www.firearmstalk.com/forums/f20/understanding-barrel-twist-bullet-weights-44557/

Lighter bullets need "more spin" than heavy bullets.
It's the other way around. Long bullets need a faster twist than shorter bullets.
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Old 09-11-2011, 11:39 AM   #6
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My understanding is it is less about weight than it is about bullet length. Naturally, a heavier bullet is longer because the diameter is constant for a given caliber. The most common way to add weight is to add length. If one were to replace the lead core with, say Depleted Uranuium (DU), one could add weight without adding length. A 55 gr lead core bullet would be the same length as a DU core bullet weighing, say 60 gr. That 60 gr DU bullet would require the same 1/12 twist barrel as the lead core 55 gr bullet. Conversly, a solid copper bullet weighing 55 gr would be significantly longer than the lead core bullet of the same weight and would require a faster twist to properly stabilize.

THe other thing that must be considered is velocity. The stabilization does not come from the twist rate, but rather the spin RPM imparted to the bullet. A 55 gr lead core bullet at 3300 fps is stable in a 1/12 barrel. If one could shoot a 60 gr bullet out of the 1/12 barrel at a higher velocity, one could adequately stabilize that 60 gr bullet in the same "slow twist" 1/12 barrel because the rpm's would be higher.

The 62 gr SS-109 bullet does not need a 1/7 twist barrel to stabilize properly. The 1/7 barrels were chosen for the military because it was needed to stabilize the much longer M-856 Tracer bullet. This bullet is longer because the core is partially filled with Red Phosphorus (much lighter than lead).

As far as "over spinning" a bullet, this is a problem only with bullets with thin jackets. Match ammo and varmint ammo tend to have thinner jackets than traditional "ball" ammo. The 55 gr FMJ BT (M-193) bullet has a relatively thick jacket and holds up well to extreme velocity. I shoot 55 gr FMJ BT's in excess of 4000 fps using sabots through a .30-06. I do not worry about the expansion characteristics of this bullet because the exteme velociy it's self is lethal. A 55 gr SP will not reach the target because it will self destruct w/in a few feet of the muzzle.
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Old 09-11-2011, 11:56 AM   #7
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Why aren't many left hand twist firearms out there? Why are firearms predominately right hand twist?
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Old 09-11-2011, 12:33 PM   #8
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Here are some Rifling configurations.
hgr_barrels_and_rifling_profile_types.jpg  
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Old 09-11-2011, 12:44 PM   #9
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Newton's 2nd Law: Rotation

The relationship between the net external torque and the angular acceleration is of the same form as Newton's second law and is sometimes called Newton's second law for rotation. It is not as general a relationship as the linear one because the moment of inertia is not strictly a scalar quantity. The rotational equation is limited to rotation about a single principal axis, which in simple cases is an axis of symmetry.
Net external torque = moment of inertia x angular acceleration
N m = kg m2 x rad/s2


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