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.