A barrel-twist rate is expressed in a chain of numbers that reflects on how far down the barrel a bullet must travel to make one full rotation. So, a 1-8 twist barrel is read as “one turn in eight inches,” and what will happen after eight inches of its journey is a 360-degree bullet rotation.
The spin imparted to a bullet by the lands or rifling is necessary to stabilize the bullet in flight. What some shooters don’t understand is that it is bullet length, not weight, that determines the amount of rotation needed for stability, though almost always it’s a bullet weight that is associated with a particular twist. The weight/length distinction didn’t matter much until the advent of the “low-drag”–style bullets. These are long, or at least longer, than other bullets of equivalent weights. Longer bullets need more rotation (higher revolutions per minute) to “go to sleep” and fly wobble-free to the target.
When it comes to choosing a twist rate, I like to err on the quicker side. When it comes to twist rate, “adequate” is a word that makes me a little nervous. I prefer “certain.” I honestly can tell you that I have not seen the ill effects on target from twist rates that were slightly faster than “adequate.” I have, however, seen bullets that needed a little tighter spiral (faster spin) to group properly. Back in the day, which wasn’t really that long ago, there were different bullets appearing to bolster the longer-range potential of .223 Rem. Most decided that 1-8 was adequate to keep an 80-grain bullet flying flat. My experience with that was more nuanced: the Sierra design at that weight, yes; the JLK, maybe. Doing load work-ups on the JLK “VLD” (Very Low Drag) bullets, which are longer than the Sierra, I had a few hit sideways as I was determining the pressure ceiling. I do that by adding a tad more propellant, watching velocities and case condition to guide this little adventure to its end. Groups tightened only when I was close to what I considered a safe maximum charge. To maintain comfort in my world, I want to see stable bullet flights from at least one big step down from “max,” and let’s say that’s a full one grain of propellant.
All in, I think a 1-7 is a better choice than 1-8 for the longer-range shooter, unless that shooter wants to try some of the 90-grain bullets, then the twist rate can be 1-6.5. And, yes, there is a limit. 1-6 blows up bullets. By the way, when a bullet comes apart, it does so from the tail forward.
The point to the foregoing was, and is, not to rely on (high) velocity for bullet stability, just get a little faster barrel twist.
A twist rate that’s too fast for “some” bullet, mostly, may subvert velocity potentials, but it’s slight, in my experience. The reason is pretty simple, as just suggested: it’s more energy needed to push the bullet through extra resistance in the bore.
The original twist rate for the AR-15 was 1-12. That’s pretty slow. It’s good for 55-grain bullets. By the way, legend has it that intentionally unstabilized bullets were the goal and reason for the 1-12. You know, the “tumbling bullets.” Hmm. 1-14 will work for those also, and that’s the Benchrest twist standard for 52-grain .224s. Didn’t happen. Barrels with 1-12 twist rates launched 55-grain mil-spec bullets in balanced harmony.
For a good while, 1-9 was the standard twist rate for most non-mil-spec barrels. Another more modern standard is 1-8. This is good for the 75- and 77-grain bullets that are commonly available in factory loads. The 1-9 hits the limit of its utility with a tangent-profile 70-grain-vicinity bullet, a 69-grain Sierra MatchKing for instance.
Some say the 1-7 twist that became standard for the A2 version of the AR-15/M16 is too fast. It’s overly fast. It’s not too fast. I’ve shot way on too many clean targets at reduced distance NRA High Power Rifle events with my 1-in-7s. For those I use either 52-grain (100 yards) or 62-grain (200 yards) bullets.
For any available factory loading I know of, which covers me in case any manufacturer does something different later on, a 1-8 twist is fine and dandy. I would advise shooters buy a barrel with 1-8 twist over one with 1-9 twist or slower just in case the shooter wants to try anything up to and including something like the 75-grain Hornady A-Max.
For .308 Win., I like 1-10. That will deal well with anything up to and including a 190-grain tangent profile, which means anything from Sierra.
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