March 3rd, 2023
6minute read
Ever wondered how bullet weights and barrel twist rates relate?
Robert A. Sadowski examines what is the best bullet weight for a 1 in 7 twist.
In the process, he teaches us the basics of everything thats involved.
Do you understand the relationship between bullets and barrel twist rates? Can you ID the right .223 bullet weight for a 1:7″ twist?Image: Adobe/Natalia80
As a bullet is fired, the rifling in the barrel forces the bullet to spin.
So, in a 1:8 twist, rate the bullet rotates one full turn every eight inches.
In a 1:7 twist, the bullet rotates one turn in seven inches.
The SAINT Edge ATC was built for maximum accuracy. It’s fitted with a Ballistics Advantage barrel with a 1:7″ twist rate.
The smaller the number, the faster the twist; you oughta remember this.
It means all the difference in stabilizing the bullet for better accuracy and terminal performance.
If a bullet spins too slowly, it cannot stabilize and wont achieve either optimum velocity or accuracy.
This is how 77-grain bullets perform in a 1:12 twist barrel; three perfect keyhole shots. The slow rifling did not stabilize the heavy bullet, causing the bullet to yaw and hit the target sideways.
What occurs is called yaw.
I built a retro AR-15 with a 20 barrel and 1:12 twist and fired 77-gr.
bullets that perfectly keyholed the target because the rifling couldnt stabilize the longer, heavier bullet.
Most general purpose AR’s have a 1:8 twist rate, like this SAINT Victor. This is a versatile twist rate capable of handling bullets from 62 to 77 grains.
So, the bullet hit the target sideways.
Accuracy is horrible with heavy bullets in that rifle.
bullets, however, that retro rifle with a 1:12 twist shoots the black out of the target.
Long-range shooting requires heavier bullets. Nosler’s 77-grain HPBT (Hollow Point Boat Tail) ammo is designed for competition and produced this 0.39″ group.
When Eugene Stoner developed the AR-15, the idea was to use lightweight bullets in the 45- to 55-gr.
range through a 20 barrel.
Barrels were rifled in a slow 1:12 twist rate, capable of stabilizing lightweight bullets but not heavier bullets.
Varmint hunting rounds like the Nosler 55-grain BTV (Ballistic Tip Varmint) can still be used successfully in a 1:7 twist barrel. This group measures 0.34″.
Fast forward a few decades, and .223 bullets have evolved in bullet style, bullet material and weight.
Today 75- and 77-gr.
.223 bullets are just as common as 55- to 62-gr.
Bullets of varying weights can be run through a 1:7” barrel. Heavier weights will be the best for the twist, while others will produce acceptable accuracy, just not optimal for the twist rate.
Twist rate is your clue on what weight bullets will perform optimally in your gun.
In fact, the sweet spot for 1:8 bores are bullets weighing from 62 to 77 grains.
In the 1980s, when the U.S. military moved to the M16A2 rifle and the 62-gr.
The change had to do with the 1:7 twist rate stabilizing heavier 70- to 77-gr.
bullets and the riflings ability to stabilize tracer rounds.
The 1:7 twist can stabilize bullets weighing up to 90 grains.
I had an engineering professor who was fond of saying, Test them like you use them.
lead-free ET rounds and the Ballistic Tip 55-gr.
BTV, to Match Grade 70-gr.
RDF (Reduced Drag Factor), and the lunker in the bunch Match Grade 77-gr.
HPBT rounds and tweaked the zeroed at 100 yards.
RDF rounds, my group opened up to 0.52.
All the heavier bullets were shooting sub-MOA.
I tried the lighter 55-gr.
all copper ET bullets provided a best group of 1.05 and deflated my balloon.
My expectations were low with the 55-gr.
Will this rifle shoot other 55-gr.
ammo with the same accuracy?
Just know, the bullet weights sweet spot for a 1:7 twist rate is 69 grains and heavier.
It will also shoot lighter bullets, but the accuracy may not be as good as it can be.
The Answer
Whats the answer to the best bullet weight for 1:7?
In short, usually heavier bullets but not always.
These are the most common twist rates on AR-15s produced today.
But remember, the SAINT Edge ATC barrel has 1:7.
Pairing the proper bullet weight with the twist rate of your ARs barrel means a better-stabilized bullet.
You still need to put in some bench time trying different ammo brands.
Think about bullet weight and barrel twist rate when choosing ammunition for your AR, and try it out.
Youll likely find a loading that your particular rifle loves.
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