Best bullet weight/length for 1-12 twist

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  1. #1
    FN Supporter Triton151's Avatar
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    Best bullet weight/length for 1-12 twist

    What is the best bullet weight/length for the SCAR 17 1-12 twist. What was it "designed" to use optimally?

    I have been doing a lot of research on my 5.56 platforms (2 AR's 1-10 twist and my SCAR 16 1-7 twist) and feel that I have a handle on which bullet length/weight is designed to shoot best for a given twist rate.

    Based on my research (the web, this site) for the 5.56, wouldn't the 1-12 twist rate for the SCAR 17 favor the lighter weight bullets? I have 2 AR 7.62's with 1-10 twist rates and am wondering if they would favor the heavier bullets over my SCAR?

    There seems to be a lot of happy shooters here using the heavier bullets through the 17. I do realize there are many factors (bullet manufacturer, type of brass, load type, individual rifle, etc).

    All things being equal, what bullet type "should" work best?

    Thanks in advance!
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    BUT, I have a 1-12" Rem 700V that shoots 180 SMK into little bitty groups

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    All things aren't equal when it comes to ballistics, and we can develop a great 100 yard load which shoots a cloverleaf, but when we move to the 600 yard line, we're in for a rude awakening. This is why I choose to shoot at the 200 yard line when developing a load for long range shooting, as the projectile is completely stabilized in flight by the time it reaches the 200 yard target.

    Food for thought...

    -SS
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    Senior Member fmsniper's Avatar
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    100% Correct------------- I always zero, check groups and run a ladder @200 yards, I run 168-175 grain in my Scar with great accuracy


    Ballistics

    The term ballistics refers to the science of the travel of a projectile in flight. The flight path of a bullet includes: travel down the barrel, path through the air, and path through a target. The wounding potential of projectiles is a complex matter. (Fackler, 1996)
    Internal, or initial ballistics (within the gun)


    Bullets fired from a rifle will have more energy than similar bullets fired from a handgun. More powder can also be used in rifle cartridges because the bullet chambers can be designed to withstand greater pressures (50,000 to 70,000 for rifles psi vs. 30,000 to 40,000 psi for handgun chamber). Higher pressures require a bigger gun with more recoil that is slower to load and generates more heat that produces more wear on the metal. It is difficult in practice to measure the forces within a gun barrel, but the one easily measured parameter is the velocity with which the bullet exits the barrel (muzzle velocity) and this measurement will be used in examples below. (Bruner et al, 2011)
    The controlled expansion of gases from burning gunpowder generates pressure (force/area). The area here is the base of the bullet (equivalent to diameter of barrel) and is a constant. Therefore, the energy transmitted to the bullet (with a given mass) will depend upon mass times force times the time interval over which the force is applied. The last of these factors is a function of barrel length. Bullet travel through a gun barrel is characterized by increasing acceleration as the expanding gases push on it, but decreasing pressure in the barrel as the gas expands. Up to a point of diminishing pressure, the longer the barrel, the greater the acceleration of the bullet. (Volgas, Stannard and Alonso, 2005)
    As the bullet traverses the barrel of the gun, some minor deformation occurs, called setback deformation. This results from minor (rarely major) imperfections or variations in rifling or tool marks within the barrel. The effect upon the subsequent flight path of the bullet is usually insignificant. (Jandial et al, 200
    External ballistics (from gun to target)

    The external ballistics of a bullet's path can be determined by several formulae, the simplest of which is:
    Kinetic Energy (KE) = 1/2 MV2
    Velocity (V) is usually given in feet per second (fps) and mass (M) is given in pounds, derived from the weight (W) of the bullet in grains, divided by 7000 grains per pound times the acceleration of gravity (32 ft/sec) so that:
    Kinetic Energy (KE) = W(V)2 / (450,435) ft/lb
    This is the bullet's energy as it leaves the muzzle, but the ballistic coefficient (BC) will determine the amount of KE delivered to the target as air resistance is encountered.
    Forward motion of the bullet is also affected by drag (D), which is calculated as:
    Drag (D) = f(v/a)k&pd2v2
    f(v/a) is a coefficient related to the ratio of the velocity of the bullet to the velocity of sound in the medium through which it travels. k is a constant for the shape of the bullet and & is a constant for yaw (deviation from linear flight). p is the density of the medium (tissue density is >800 times that of air), d is the diameter (caliber) of the bullet, and v the velocity. Thus, greater velocity, greater caliber, or denser tissue gives more drag. The degree to which a bullet is slowed by drag is called retardation (r) given by the formula:
    r = D / M
    Drag is difficult to measure, so the Ballistic Coefficient (BC) is often used:
    BC = SD / I
    SD is the sectional density of the bullet, and I is a form factor for the bullet shape. Sectional density is calculated from the bullet mass (M) divided by the square of its diameter. The form factor value I decreases with increasing pointedness of the bullet (a sphere would have the highest I value).
    Since drag (D) is a function of velocity, it can be seen that for a bullet of a given mass (M), the greater the velocity, the greater the retardation. Drag is also influenced by bullet spin. The faster the spin, the less likely a bullet will "yaw" or turn sideways and tumble in its flight path through the air. Thus, increasing the twist of the rifling from 1 in 7 will impart greater spin than the typical 1 in 12 spiral (one turn in 12 inches of barrel).
    Bullets do not typically follow a straight line to the target. Rotational forces are in effect that keep the bullet off a straight axis of flight. These rotational effects are diagrammed below:
    Yaw refers to the rotation of the nose of the bullet away from the line of flight. Precession refers to rotation of the bullet around the center of mass. Nutation refers to small circular movement at the bullet tip. Yaw and precession decrease as the distance of the bullet from the barrel increases.
    What do all these formulae mean in terms of designing cartridges and bullets? Well, given that a cartridge can be only so large to fit in a chamber, and given that the steel of the chamber can handle only so much pressure from increasing the amount of gunpowder, the kinetic energy for any given weapon is increased more easily by increasing bullet mass. Though the square of the velocity would increase KE much more, it is practically very difficult to increase velocity, which is dependent upon the amount of gunpowder burned. There is only so much gunpowder that can burned efficiently in a cartridge. Thus, cartridges designed for hunting big game animals use very large bullets.
    To reduce air resistance, the ideal bullet would be a long, heavy needle, but such a projectile would go right through the target without dispersing much of its energy. Light spheres would be retarded the greatest within tissues and release more energy, but might not even get to the target. A good aerodynamic compromise bullet shape is a parbolic curve with low frontal area and wind-splitting shape. The best bullet composition is lead (Pb) which is of high density and is cheap to obtain. Its disadvantages are a tendency to soften at velocities >1000 fps, causing it to smear the barrel and decrease accuracy, and >2000 fps lead tends to melt completely. Alloying the lead (Pb) with a small amount of antimony (Sb) helps, but the real answer is to interface the lead bullet with the hard steel barrel through another metal soft enough to seal the bullet in the barrel but of high melting point. Copper (Cu) works best as this "jacket" material for lead.






    what does all of this mean/
    some bullets in many rifles do not stabilize @ 100 yards, but normally will @200
    Last edited by fmsniper; 11-09-2012 at 12:39 PM.
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    Senior Member Xn0r's Avatar
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    Whoa! Put that into a PDF! Great info!

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    From what I have found and played with. a 168gr. BTFMJ sitting on top of 42gr of Varget is the perfect round. At least for me.. Performs very well..

  7. #7
    Senior Member fmsniper's Avatar
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    I run American Eagle 168 OTM, but at longer ranges like 600 meters + I use M118 ammo
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    Member Gark's Avatar
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    Best bullet weight/length for 1-12 twist

    I am partial to 168gr BTHP for accuracy
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    Senior Member Bullseye Shooter live4speed's Avatar
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    Considering the SCAR 17 (MK 17) was designed for the SOCOM boys, I would think FN would design the rifle to shoot the current mil-issued loads as well as possible. The 1/12 twist does tend to perform VERY well with 168gr rounds, but I dont think that was the primary reason for selecting that twist. From what i understand the 1/12 is the most versatile twist rate for a .308 allowing operators to shoot the 130 gr MK 319 Mod 0, the 149gr M80 ball, or any of the larger 170+ grainers with relative accuracy.
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  10. #10
    Junior Member deltablack's Avatar
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    Twist rate affects the stability of the bullet, because of that you may see some issues if you go beyond the 175gr projectiles. The lower velocity of the 16" barrel and the slow twist rate is going to have a tough time with the heavies. I wouldn't be surprised if the best accuracy would be had by bullets in the 130-155gr range.

    Accuracy is the sum of many parts in a firearm, not just twist rate. If a projectile destabilizes, it is not beneficial for accuracy, but as long as it is stable, there are many other factors that play a larger role in accuracy.
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