Best Bullet Weight for 1 in 7 Twist Rate?

BY PGF
11 months, 1 week ago

I thought readers might find some interest in this.

As a bullet is fired, the rifling in the barrel forces the bullet to spin. So, in a 1:8” twist, rate the bullet rotates one full turn every eight inches. In a 1:7” twist, the bullet rotates one turn in seven inches. The smaller the number, the faster the twist; you need to remember this.

[…]

If a bullet spins too slowly, it cannot stabilize and won’t achieve either optimum velocity or accuracy. What occurs is called yaw. The bullet is unstable and does not hit the target with the tip of the bullet, but perhaps the side of the bullet.

I built a retro AR-15 with a 20” barrel and 1:12” twist and fired 77-gr. bullets that perfectly keyholed the target because the rifling couldn’t stabilize the longer, heavier bullet. So, the bullet hit the target sideways. Accuracy is horrible with heavy bullets in that rifle. With 55-gr. bullets, however, that retro rifle with a 1:12 twist shoots the black out of the target. Rifling can also be too fast and over-stabilize the bullet causing the bullet to fragment in flight and lose all effectiveness.

When Eugene Stoner developed the AR-15, the idea was to use lightweight bullets in the 45- to 55-gr. range through a 20” barrel. Barrels were rifled in a slow 1:12” twist rate, capable of stabilizing lightweight bullets but not heavier bullets. Fast forward a few decades, and .223 bullets have evolved in bullet style, bullet material and weight. Today 75- and 77-gr. .223 bullets are just as common as 55- to 62-gr. bullets. Twist rate is your clue on what weight bullets will perform optimally in your gun. Some shooters might not think twice about the twist rate in their barrel, but if they knew that could fine-tune their bullet performance they might pay closer attention.

Twist Rate Sweet Spot

Most AR-15 rifles and carbines produced today use rifling with a 1:8 twist rate. In my opinion, a twist rate of 1:8 is perfect for a general-purpose, 16” barrel AR since this twist offers versatility and can easily stabilize both light and heavy bullets. In fact, the sweet spot for 1:8 bores are bullets weighing from 62 to 77 grains.

In the 1980s, when the U.S. military moved to the M16A2 rifle and the 62-gr. M855 cartridge, it chose a 1:7 twist rate that has become the de facto rifling in all U.S. military rifles and carbines chambered in 5.56 NATO. The change had to do with the 1:7 twist rate stabilizing heavier 70- to 77-gr. bullets and the rifling’s ability to stabilize tracer rounds. The 1:7 twist can stabilize bullets weighing up to 90 grains.

I had an engineering professor who was fond of saying, “Test them like you use them”. So, to prove out the thesis, I sat down at the range bench with a stock, off-the-shelf Springfield Armory ATC with its 1:7 twist rate for heavy bullets and mounted with a Leupold Patrol 6HD 1-6x24mm scope. I used Nosler cartridges since they provide a wide assortment of bullet weights, bullet material and bullet types — from lightweights like the Expansion Tip 55-gr. lead-free ET rounds and the Ballistic Tip 55-gr. BTV, to Match Grade 70-gr. RDF (Reduced Drag Factor), and the lunker in the bunch Match Grade 77-gr. HPBT.

More, including test results, at the link.


Comments

  1. On March 22, 2023 at 8:53 pm, X said:

    Retarded cops lose rifle off roof of four-story building during parade:

    https://www.audacy.com/wben/news/local/photos-buffalo-police-sniper-rifle-falls-from-roof-during-st-patricks-day-parade

  2. On March 23, 2023 at 9:51 am, Scott in Phx said:

    This is what I love about “the best rifle evah”(tm)

    All this mind numbingly arcane discussion of subjects requiring a rudimentary knowledge of physics.

    I miss that over in the M14 blogs

  3. On March 23, 2023 at 5:16 pm, Georgiaboy61 said:

    In considering which barrel twist rate to use, it is important to remember that bullet weight is only one of several variables which impacts the performance of that projectile.

    Many industry people consider Bryan Litz of “Applied Ballistics” to be one of the foremost ballisticians working today. Litz is not only the owner/founder of AB, he is also an aeronautical engineer, head ballistician for Berger Bullets, and a seasoned competitive shooter.

    As he notes in his writings and on his website, bullet length is also very important in calculating the twist rate needed for optimum stability in flight, and in addition bearing surface (that part of the bullet which actually engages the rifling in the bore). Solid copper bullets such as those offered by Barnes and Hornady, for example, often need more-aggressive than expected twist rates, most likely due to variances in bearing surface compared to traditional designs.

    Berger’s website and maybe also that of Applied Ballistics as well, has a bullet stability calculator, it is germane to note…. for those interested in calculating such things or who are uncertain if they have selected the proper bullet and twist-rate combination.

    Getting back to the U.S. military’s adoption of the 62-grain SS109/M855 round in the early 1980s, I have always wondered why the army opted for a 1:7 twist rate when a 1:8 would have worked just as well.

    In other words, they had been considering a 1:9 rate prior to stability problems with the new 64-grain tracer ammunition which was longer than the standard ball projectile, whereupon the 1:7 twist was selected to solve the problem. Why not a 1:8 twist instead? If anyone knows the backstory on that decision, it would be interesting to learn it.

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This article is filed under the category(s) Ammunition,AR-15s and was published March 21st, 2023 by PGF.

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