We'll start with some information from bullet sellers. This data is actually compiled from twist rates suggested for individual bullets. Since the length of the bullet, more than the weight is the main issue in determining the required barrel twist these numbers should not be considered applicable to all bullets currently on the market. For example, although Berger recommends a 1:9 twist for 70 grain bullets in general, they do sell some 70 grain bullets that they state will stabilize in a 1:10 twist barrel.
Bullets made of solid copper (or gilding metal or bronze), or bullets with steel cores will be longer for a given weight since those metals are less dense than lead. Because of their additional length, they might not stabilize with the twists recommended for a conventional (copper-jacketed lead core) bullet of the same weight. However, it's safe to consider this general information to be a good starting point.
It is worth remembering that bullet sellers are likely to recommend a faster twist than a slower twist if there's any question about what will work since they want satisfied customers. There's some real world shooting data at the end of the post that demonstrates that their recommendations tend towards the conservative.
Of course, a faster twist (smaller second number--1:7 is a faster twist than 1:8) will always stabilize lighter (shorter) bullets than listed in the charts.
It's also important to keep in mind that higher muzzle velocities will result in a faster spin for a given rifling rate. So a bullet that might be marginally stabilized in a 14" barrel might stabilize just fine out of a 26" barrel with the same twist because of the higher muzzle velocity.
Here's a plot that demonstrates the relationship between muzzle velocity and bullet spin rate. Remember that bullet spin rate is measured in revolutions per second while rifling twist is a measure of how far forward the bullet moves during each revolution. The two things are not the same although they are obviously related.
Condensed from the old Berger website.
.224 (.223) bullet weight vs barrel twist.
30-45 Grains 1:15 TwistHere's a FAQ answer from the new Berger website
50-55 Grains 1:14 Twist
60-64 Grains 1:12 Twist
70-75 Grains 1:9 Twist
80 Grains 1:8 Twist
Spinning a bullet faster than necessary can amplify any inconsistency in the bullet. Since we use J4 jackets, you can shoot Berger Bullets in faster twist than what is listed. We list the slowest twist rate needed because we want to squeeze every bit of accuracy out of a rifle.From Sierra's website
45 Grains 1:16 TwistGroup Size vs. Twist Rate Data
50-55 Grains 1:14 Twist
60 Grains 1:12 Twist
65-69 Grains 1:10 Twist
77-80 Grains 1:8 Twist
90 Grains 1:6.5 Twist
Ok, now for something a little more visual to work with. I compiled the results from two tests. One test was conducted by R. L. Window and Dick Metcalf for an article in Shooting Times Magazine and the other by James Tarr for an article in Guns & Ammo Magazine.
In all, the two tests used 9 rifles with 4 different rifling twists and 17 types of ammunition with 10 different bullet weights.
Here are the results. Notice that even the 1:12 twist stabilized bullets up to 69 grains but that above 69 grains, the bullets are unstable in the slower twists. The 90 grain bullets required at least 1:8 twist to stabilize them and the 77 grain bullets needed at least 1:9 twist.
The reason the lines aren't continuous is that not all bullet weights were tested with all the twists. For example, there was no data for 75grain bullets in a 1:12 twist barrel so there's a break in the line for the 1:12 twist data at the 75 grain point.
Please note that the minor differences in average group size are merely variations in the data. Don't try to analyze those variations as having significant meaning in terms of which bullet weight will shoot most accurately for a given twist.
The important information is whether the group sizes are acceptable (under 3" or so--which means the bullets were properly stabilized) or whether they're unacceptably large (larger than 3"--which means that the bullets were not stabilized).
The exact group size is not important. The important information is: Are the groups really large (unstabilized bullets) or are they acceptable (stabilized bullets).
Here's the same data from a slightly different perspective. There are two different plots, one shows 7 of the bullet weights--the ones tested in all
of the 4 rifling twists that were used in the test. The second one has the remaining 3 bullet weights that were tested in only 2 of the rifling twists used.
I tried to make one plot with all of the bullet weights, but it was misleading due to the fact that not all bullet weights were tested in every rifling twist, so I had to break it into two plots to avoid misunderstandings.
It's worth keeping these graphs in mind. One thing that they show is that in the test rifles, 1:9 twist worked for even 77 grain bullets which are about the top limit of what will fit in an AR magazine. All 3 of the 1:9 twist rifles tested shot the 77 grain bullets well. In other words, in these tests, any .223/5.56 loading that would fit in a typical .223/5.56 magazine were stabilized by a 1:9 twist under the test conditions.
Even the much-maligned 1:12 twist stabilized bullets weighing up to 69 grains.
Keep in mind that the shortest barrel length used for testing 1:9 twist was 20" and the testing was carried out at 100 yards. At longer ranges, or with shorter barrels, or when other conditions significantly reduce muzzle velocity, stabilization problems may occur with very heavy bullets (75 grains and up) when using 1:9 twist.
The following thread provides some input from shooters who have had stabilization issues with short barrels and/or low temperatures, both of which can reduce muzzle velocity and therefore bullet rpm.