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another dumb ar question

stayathomedad
What is there differnt rifling? It seams that the most common is 1:9 but i have seen others. like 1:8 1:12
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  • p3skyking
    Different rifling for different bullet coefficients and weights. Lots of ways to impart spin and the frequency of the spin. The goal is a stable inflight profile of the projectile.
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  • beantownshootah
    quote:Originally posted by stayathomedad
    What is there differnt rifling? It seams that the most common is 1:9 but i have seen others. like 1:8 1:12


    Well, to make this simple, different rifling twist rates are used to stabilize different weight bullets.

    In general heavier (longer) bullets need a faster rate of twist (eg 1:7), where as lighter bullets can use a slower rate of twist (eg 1:12).

    If you fire a really heavy bullet through a slow-twist barrel, the bullet won't spin enough to stabilize it. So it will wobble in flight, or even worse actually tumble end over end in flight, leading to both poor accuracy and poor terminal performance.

    Conversely if you take a really light bullet and overspin it through a super-fast twist rate barrel, you'll also get less than ideal accuracy. In fact, in extreme cases, the bullet itself will shear apart due to centrifugal force from spinning so fast!

    1:9 twist rate is probably currently the most popular as its a good compromise that works reasonably well on the most common bullet weights (eg 55 and 62 grains).

    Part of this proliferation of many barrel twist rates comes with the increasing popularity of AR-15 type guns for all sorts of different applications. For example, the people using them for match competition at several hundred yards want to use super-heavy bullets that the gun wasn't originally designed for. Ditto for hunters who are after larger game like deers, etc.
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  • armilite
    Back in the 60's when the AR's first came out the 55 grain bullet was about all that could be had thus the 1 in 12 twist rate stabilized the 55 grainers. Since then a proliferation of bullet weights have been introduced thus the change in rifling twist. Nowadays a 1 in 7 or 1 in 8 gives the best performance across the board. Fine tuning with different bullets can bring the maximum performance out in a barrel.
    I used to have a .223 Contender 1 in 12 inch twist that with certain bullets they would go through the paper target sideways.
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  • sandwarrior
    quote:Originally posted by armilite1015
    Back in the 60's when the AR's first came out the 55 grain bullet was about all that could be had thus the 1 in 12 twist rate stabilized the 55 grainers. Since then a proliferation of bullet weights have been introduced thus the change in rifling twist. Nowadays a 1 in 7 or 1 in 8 gives the best performance across the board. Fine tuning with different bullets can bring the maximum performance out in a barrel.
    I used to have a .223 Contender 1 in 12 inch twist that with certain bullets they would go through the paper target sideways.


    Remember when the Nosler 60 gr. Partition was "THE" heavy in .22 cal bullets? That would just barely stabilize in a 1-12" twist out of a .222. Our 1-14" twist wouldn't stabilize it.
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  • nononsense
    stayathomedad,

    Selecting the proper twist rate for a barrel, cartridge and bullet combination is based on the bullet's length and the velocity range. The smaller the number (1:7") is considered to be a fast twist which is suited to the very longest and heaviest bullets such as the 77, 80, 85 and 90 grain VLD Match bullets. I have standardized on the 1:7.7" for these bullets which is optimal for the velocity ranges used.

    Slow twists such as the 1:14" are really suited to the varmint bullets which are short and light. The shorter bullets don't require as much twist to achieve stability.

    If you are unsure of which twist to use, read the chart below or use the calculator link to help you with that decision. Remember that the best accuracy is achieved from the slowest twist rate necessary to stabilize the bullet within a given velocity range.

    Twist Rate for Barrel?

    What is the right twist rate for an AR15/M16 barrel? The below is from Corey Sattler of Olympic Arms. It will give you a good idea of what you should look at.

    * 1x7
    Technically too tight of a twist for any .224 bullet widely available, including the 80 grain. Manufactured originally to stabilize SS109 and/or tracer ammo, but further military testing has shown other twists to be superior. Some of the very heavy custom stuff (87 grain?) may work well, but why?

    * 1x8
    Great twist for 69-80 grain bullets. Sierra once informed me that the perfect twist for their 80gr was 1x8.2xxxxx (blah blah blah big long engineers explanation followed).

    * 1x9
    Good all around twist ratio. Best suited for 52-69 grain, but either end of the envelope will be questionable.

    * 1x10
    Practically the same as 1x9, but favors the lighter side a little more.

    * 1x12
    Great for the 40-52 grain bullets. Most often found on bolt action rifles as their primary use is varminting. All right for the 55 grain, but not the best.

    * 1x14
    If you want to under stabilize the .224 bullet, use this twist. Useless except for tumbling effect it causes upon bullet impact. IMHO, better to shoot straight with a good HP bullet as your chances to hit are better and damage will most likely be greater also.

    http://www.triplebreakproducts.com/FAQ/twist_rate_for_barrel.htm

    This is a good simple article:

    http://www.shootingtimes.com/ammunition/st_twistrates_200809/

    This is a good semi-technical explanation:

    http://www.loadammo.com/Topics/July01.htm

    A plug & play twist rate calculator:

    http://www.uslink.net/~tom1/twistrate.htm

    Best.
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