Barrel twist ratios

Discussion in 'Technical Questions & Information' started by snipe1957, Sep 18, 2009.

  1. snipe1957

    snipe1957 New Member

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    I have an interest in the M&P15vtac(1 in 7),M&P15pc(1 in 8) and M&P15t(1 in 9).I have heard that barrel twist can affect accuracy and that heavier longer bullets do better with a higher ratio (1 in 7) and lighter shorter bullets do better with a lower ratio (1 in 9).But I can't find specifics.I like everything about the vtac,but most economical and easy to find .223 ammo is around 55 grain.So does anyone have any experience with MOA characteristics of .223,55grain ammo in the M&P15VTAC or in a rifle with a 1 in 7 twist compared to 62+ grain ammo?
  2. snipe1957

    snipe1957 New Member

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    OK, with help from the people at Smith & Wesson,I am going to answer my own question.According to those guys,the difference in MOA between a 52g and a 75g round at 300 yards in a 1 in 7 twist barrel is approximately 1".I take this to mean that if the heavy round holds a 1"MOA then the light round should hold approximately a 2"MOA.Conversely,the difference in MOA between a 52g and a 75g round at 300 yards in a 1 in 9 twist barrel is approximately 1" with the light round being the most accurate.This is verbal information provided by Steve and his colleges at Smith & Wesson customer service.He says these twist are designed around the sweet spot of the .223,52g to 75g, round.
  3. fprefect

    fprefect New Member

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    I don't think I've heard it described exactly like that before, but I shall not argue with the experts and S&W. The faster a bullet leaves the bore, the less twist is required to stabilize the bullet out to it's maximum useful range. With some "hot" 22 centerfire cartridges bullets will be turning at close to 400,000 rpms when leaving the barrel and twist of 1 in 14 or 16 is all that is required while the heavier bullets, which are typically used for longer range shooting leave the bore at a slower velocity and my require a 1 in 9 or even 1 in 8 to keep them stable at longer ranges.

    There is a table at http://www.nfa.ca/content/view/129/197/ that will give you some idea of what I'm attempting to describe.

    However the "tighter" twists that are required for heavier bullets also make the bullet more difficult to push down the bore and can therefore produce higher pressures. I think this is possibly what Steve at S&W may have been referring to as the "sweet spot". Less twist to produce higher velocities with lower pressures for lighter bullets, yet a "tight" enough twist to stabilize the heavier bullets for targets outside the range of a lighter bullet that will lose velocity much more rapidly and can be "moved" by considerably distances by even a 5 mph crosswind. At least that's my take on it.

    F. Prefect
  4. snipe1957

    snipe1957 New Member

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    Thanx for the link to the twist rate chart,Prefect,I'll bookmark that.
    Yea,my concern was that the lighter rounds would become unstable due to the high rate of spin and decrease accuracy,which apparently it does but not significantly(1" greater MOA at 300 yards).I think all rounds,like anything in motion, have an optimum spin(RPM),and going to high above or to for below that "sweet spot" produces instability.
    I guess Smith & Wesson is further specializing the M&P15 by making models with different twist ratios to fine tune them for long range (300+yrds) shots with different bullet weights.
    Knowing this I can feel good about getting the VTAC and using 55g ammo for average shooting,But if I want to shoot flies off a fence post at 300 yards I will need to use 75g ammo with the 1 in 7 twist.If someone wants to shoot those flies with 55g ammo the 1 in 9 barrel models are more appropriate
  5. LDBennett

    LDBennett Well-Known Member

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    Here's a real life experience (mine) with barrel twist in an AR15:

    My AR15 has a 1 in 8 inch twist barrel. I loaded up my favorite 52 grain Speer bullets (hunting grade, NOT match). The bullets never made it to the target. The high rotational speed literally spun them apart. When I went to 52 grain Match bullet they would get to the target with just OK accuracy. I then switched to 75 grain Hornady Match bullets and the accuracy became great! The problem according to experts is the hunting bullets are not as uniform in where its center of mass is compared to the center of rotation. The match bullet are more uniform. That's why the Match 52 grainer stayed together

    You have to match the bullet LENGTH (not weight) to the barrel twist. Given the same bullet caliber a longer bullet requires more twist to stabilize it in flight. There is no scientific reason that you can't over spin a bullet beyond the minimum as determined from the Greenhill formula but the lighter (and consequently shorter) bullets may not be constructed well enough to stand the very high rotation speeds.

    If you get a 1 in 7.5 or 1 in 8 twist 223 barrel then use only the bullet weight (length) that the twist rate supports. Use the bullets that the barrel was made to handle.

    I fought with this and got good advice from several experts here and the bottom line is use the right bullets for the twist of the barrel. Anything else MAY be a compromise in accuracy or reliability of the bullet. In gerenal, don't plan on using a 55 grain bullet in a 1 in 7.5 inch barrel, without paying a price.

    LDBennett
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