The Modern AR-15.
is always a Controversial topic - what barrel with what twist rate is
the best to use. The answer is pretty simple - depending what you will be
using your rifle for.
1-in-9 is a great compromise twist rate—not too fast to cause problems with the 55-grain Bullets, but fast enough to stabilize all but the heaviest bullets under most circumstances. This twist will stabilize most traditional bullets up to 75-grains, and monolithics up to 70-grains—but they do so right at the edge of the envelope so not all rifles will do it.
For a 16-inch general-use carbine, the 1-in-8 twist is about as versatile as it gets. This twist rate will comfortably stabilize bullets up to 80-grains, and the excellent 75- and 77-grain bullets also work great at a wider spectrum of velocities—which means barrel length isn’t critical. You will commonly see a 1-in-8 twist in .223 Wylde chamberings, and it is popular among 3-gun and and other competition shooters. 1-in-8 twist happens to by my personal choice for a go to twist rate. 90% of the time i'll plink with 55gr, and occasionally switch to a 70+ gr bullet for hunting.
This is the twist chosen by the military since the switch was made to the M16A2—and the 62gr. M855 cartridge—in the 1980s. This twist is found on the M4 carbine, the M16A4, the Mk12 Special Purpose Rifle and even the HK416. Its ability to stabilize tracer rounds in-flight is one of the reasons that the military chose this twist rate. Because the Military went with a 1-in-7 twist rate, if is VERY common to see it on the civilian built AR-15's. Whatever the Military goes with will usually become popular shortly after.
This barrel will stabilize bullets of up to 90 grains, and can handle the 70- to 77-grain bullets at just about any velocity, which makes it well suited for carbines or AR Pistols with very short barrels. If you want a Mil-Spec clone, the 1-in-7 twist is the way to go.
There are many factors when it comes to shooting accurately. Matching the twist rate in your rifle or pistol to the appropriate ammunition won’t guarantee great accuracy, but it will ensure the bullet is properly stabilized in flight. For instance with a 1-in-8 or 1-in-7 twist rate rifle the 62gr M855 will usually shoot larger groups than 55gr M193. This is due to the steel core ever-so-slightly of center and differ from round to round; even though the twists are more appropriate for the 62gr, the construction of the bullet led to greater inconsistencies. On the other hand, using a bullet that’s too heavy for your barrel’s twist is a virtual promise of poor accuracy and ineffective terminal performance. If you’re struggling with the accuracy of your modern sporting rifle, be sure you’ve properly matched your ammunition to the barrel’s twist. If you are a long range shooter planning on building a precision rifle with a 30" bull barrel; I would want to plan out what my goals are, what I ammo I intend to shoot, and go from there. For the MAJORITY of shooters out there the most commonly available ammo is some variation of 55gr, for this purpose 1:9, 1:8 or 1:7 will suit you just fine.