One of the things that is overlooked by people who have begun buying into the AR-15 is buying the proper ammo for the twist rate of your barrel. Ar-15 barrels that are chambered for 5.56mm come in various lengths and twist rates, with the most common being 1:6, 1:7, 1:8, and 1:9.
Like throwing a football, the rifling of a barrel causes the bullet to spin around its long axis in order to stabilize it in flight. This rate is called "twist rate." Twist rate is the number of inches the bullet travels in order to complete a full rotation and is expressed as a ratio. In other words, a 1:8 twist rate will cause the bullet to spin once every 8 inches of travel. A 55 grain bullet leaving the barrel at 3,240 feet per second will be spinning at nearly 300,000 rpm.
Since muzzle velocity is a function of pressure, which in itself is related to bullet weight, the weight of the bullet dictates which twist rate is appropriate for each bullet. The twist rates on all but one of my AR-15s is 1:7, even though 1:9 is the more popular barrel. The fast twist rate favors slower (in other words, heavier) bullets. The ideal twist rates for bullets is:
Weight: 40-Grain Twist Rate: 1:12 to 1:9
Weight: 55-Grain Twist Rate: 1:9 or 1:8
Weight: 62-Grain Twist Rate: 1:8 or 1:7
Weight: 77-Grain Twist Rate: 1:7 or 1:8
Weight: 80-Grain Twist Rate: 1:7
Note that the 1:7 twist is best for 62 grains and higher. This is the reason why I favor the Lake City Green Tip, which is a 62 grain bullet.
If I had a barrel with a 1:9 twist rate, then a better choice would be the 55 grain bullet. This bullet is faster and works better with a longer twist rate.
Why did I pick that particular twist rate? Lighter bullets do better at ranges under 100 yards. The heavier bullet is better for accuracy at ranges over 100 yards. Honestly, I would have chosen a 1:8 twist, but I couldn't find one at the time.
At ranges under 100 yards, the lighter and faster 55 grain bullet is better at defeating body armor.
At ranges over 100 yards, the heavier 62 grain bullet is now travelling at the same velocity as the lighter one, meaning that it has more energy at longer range, and being steel, more penetrating ability.
Hope this helps.
5 comments:
When looking for a barreled upper it has nearly impossible to find anything except 1:7
Therefore: I had found the same thing. It's not too difficult to rebarrel your upper, if you find one you like.
Generally, I thought the twist rate related more to the aspect ratio of the projectiles, with long-and-thin needing a faster rate than "stubbier" projectiles. In the case of a bullet, with a fixed caliber, and similar materials (e.g. lead) used to make it, heavier bullets winds up having to be longer than light bullets, thus having a larger aspect ratio, and needing a faster twist rate than a lighter (and thus stubbier) projectile.
You are correct that it is the length of the projectile. I used bullet weight because it is directly related to the length of the bullets that are the same caliber. When you buy ammo, the length of the bullet isn't specified, the weight is. Therefore, it is easier to think of it as heavier versus lighter.
However, it is the heavier bullet requires a slower rotation and hence a lower twist rate.
Also, the 62gr 5.56 bullet has that steel core. Steel is less dense than lead. Therefore you need more volume. You can't make the bullet larger (duh) so it has to get longer, which changes case volume as well
I haven't rebarreled an upper yet. It is on my list of fun things to do. I want to rebarrel am upper to 14.5" with A2 style gas block. That makes it an expensive barrel with the $200 tax stamp
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