Twist rate question

[Sailormilan2]

I'm getting ready to load some 77 gr CC bullets for my .223. I've never loaded them before, so I need to ask a question re twist rate before I get started. I have 2 uppers, one with a .223 Wylde chamber, 20" long with a 1 in 8" twist, and one with a 5.56 NATO chamber, also 20" long, but with a 1 in 7" twist. All other things being equal, which would work better with the 77 gr bullets? I'm pretty sure that if the 1 in 8" barrel was a 16", it would not handle it. But with the extra length giving me more velocity, will it work? TIA.

[kcstott]

Have you ever used Berger bullets stability calculator? All questions can be found there. Be honest with the data you input and you will get an honest answer.

[Sailormilan2]

Well, I don’t have the bullets yet, so I can’t measure the length. Since twist is a function of length and velocity, I was just trying to get an idea.

Edited:
Bullets showed up today. They're 1.006" long, and it appears that as long as they're doing 2800 fps, 1 in 9" will just barely work. So, 1 in 8" should be fine.

[ar15barrels]

I'm getting ready to load some 77 gr CC bullets for my .223.
I've never loaded them before, so I need to ask a question re twist rate before I get started.
I have 2 uppers, one with a .223 Wylde chamber, 20" long with a 1 in 8" twist, and one with a 5.56 NATO chamber, also 20" long, but with a 1 in 7" twist.
All other things being equal, which would work better with the 77 gr bullets?
I'm pretty sure that if the 1 in 8" barrel was a 16", it would not handle it.
But with the extra length giving me more velocity, will it work? TIA.

If the barrels are of equal quality, the 1:8 should be slightly more accurate than the 1:7" twist.

A 16" 1:8 will handle 77's just fine.
A 20" 1:9 will also handle 77's just fine.

[Sailormilan2]

If the barrels are of equal quality, the 1:8 should be slightly more accurate than the 1:7" twist.

A 16" 1:8 will handle 77's just fine.
A 20" 1:9 will also handle 77's just fine.

Thanks AR.

[acourvil]

Why would faster twist be less accurate? Or was the 7 supposed to be a 9?

[TomReloaded]

If you're using energy to excessively spin something, it's going to slightly wander off a straight path... Not a lot, but technically the slower twist will be more accurate as long as it's stable.

But really, you *probably* won't notice, and tons of factors exist otherwise that might make the 7 twist more accurate. Or less. The best quality barrel will come out ahead, but if all things are equal you do want minimal spin.

[ar15barrels]

Why would faster twist be less accurate?

ALL bullets are imbalanced to some amount.
Cheaper bulk blaster bullets have more imbalance than high quality target bullets.
The faster the twist, the MORE effect that the imbalance has on group size.
So by using the slowest possible twist rate, you are minimizing the accuracy that would be lost to excess twist rate because of the bullet imbalance.

Benchrest shooters figured this out decades ago so that's why they routinely run 11", 12", 13" or 14" twists as appropriate to their bullets and elevation.
You can run slower twists when the air is thinner and higher elevation automatically gives you thinner air so people shooting in Denver can get away with slower twists than people shooting in Death Valley.

[ar15barrels]

But really, you *probably* won't notice, and tons of factors exist otherwise that might make the 7 twist more accurate.
Or less. The best quality barrel will come out ahead, but if all things are equal you do want minimal spin.

The difference in group sizes from blaster grade ammo in a 1:12" twist vs a 1:7" barrel is quite obvious.
Go test it yourself with any of the cheap 55gr 223 ammo that people use because it's cheaper.

The only time a 1:7" is more accurate than a 1:9" twist of equal quality barrels/bullets is when you use a bullet length that the 1:9" won't stabilize.

[cz74]

Use the berger stability calculator https://bergerbullets.com/twist-rate-calculator/
You can manually enter your data points if you don't find your bullet in their dropdown list.

[jimmykan]

I don't think the twist rate is the biggest factor in AR barrel accuracy.

All the the stuff said above is true, about spinning an imbalanced bullet faster will make it less accurate.

The chamber and throat dimensions play a role in this too. If the throat has a lot of clearance before the rifling begins, there is a greater chance that the bullet becomes slightly cocked off-axis as it enters the rifling. Off-axis spin will also make a bullet less accurate. And if the bullet is also imbalanced, then off-axis spin just make the imbalance even worse.

But you say that you're shooting Nosler Custom Competition 77's. I shoot these too, and they are good consistent bullets, the inherent imbalance in these will be negligible in the context of shooting them out of an AR.

From a practical standpoint, an AR barrel's twist rate is either fast enough to stabilize the Nosler 77 bullet and you'll be able to shoot groups, or it will not be fast enough to stabilize that bullet and you will get crazy fliers and keyholes in the target with no discernable grouping to be seen.

1-8" twist and faster will definitely be fast enough. 1-9" will be marginal, depending on the muzzle velocity and atmospheric conditions. Slower twists like 1-12" will not stabilize a 77-grain bullet at 5.56 velocities, unless you're shooting at 5000' ASL in 100 degree weather (a.k.a. Montana right now).

I have a 1-9" twist 18" (5.56NATO chamber) Criterion chrome-lined barrel that will shoot the Nosler 77 into groups just over an inch at 100 yards.

I have a 1-8" twist 20" (223 Wylde chamber) AR Stoner (MidwayUSA bargain brand) stainless barrel that will shoot the same bullet into 1.5-2 inch groups at 100 yards.

I have two 1-7" twist 20" (223 Wylde chamber) White Oak service rifle barrels that will shoot the same bullet into groups just under an inch at 100 yards. These are the most accurate ARs that I own.

I have a 1-7" twist 14.5" (5.56NATO chambered) Bravo Company hammer forged lightweight barrel that shoots the same bullet into 3 inch groups at 100 yards and that's the best it can do.

[Grobie]

I'm getting ready to load some 77 gr CC bullets for my .223. I've never loaded them before, so I need to ask a question re twist rate before I get started. I have 2 uppers, one with a .223 Wylde chamber, 20" long with a 1 in 8" twist, and one with a 5.56 NATO chamber, also 20" long, but with a 1 in 7" twist. All other things being equal, which would work better with the 77 gr bullets? I'm pretty sure that if the 1 in 8" barrel was a 16", it would not handle it. But with the extra length giving me more velocity, will it work? TIA.

Hope this helps

[TomReloaded]

The difference in group sizes from blaster grade ammo in a 1:12" twist vs a 1:7" barrel is quite obvious.
Go test it yourself with any of the cheap 55gr 223 ammo that people use because it's cheaper.

The only time a 1:7" is more accurate than a 1:9" twist of equal quality barrels/bullets is when you use a bullet length that the 1:9" won't stabilize.

He had only an 8 and 7 twist, so my comment was more about that. 8 and 7 is pretty close. Without knowing what the barrels are, i wouldn't confidently guess which of the two is better.

I think most people would be better off with a 9+ twist barrel. But 7's are popular, so we basically all have them now. It's an annoying default.