I am contemplating buying an AR with a 1:8 twist,

What weight of bullets should that twist rate be able to shoot 1" or better groups at 100 yards?

Thanks

The only way you'll know is by actually shooting different bullets.

If I understand it correctly, the theory is that a fast twist barrel will shoot heavier/longer bullets more accurately than a slower twist barrel would shoot the same bullets.

And vice versa: according to the theory, a slower twist barrel should shoot lighter/shorter bullets better than a faster twist barrel would shoot the same bullet.

I have a 6mm (1:12) which keyholes bullets that are too long/heavy (spitzers above 90 grains). It lives by the rule.

On the other hand, I have a .223 (1:9) that is sub-MOA with short/light bullets (50 grain). It does not live by the rule.

Just remember what the philosopher Yogi Berra, former catcher for the NY Yankees taught us: "In theory, theory and practice are the same. In practice, they are not."

I have a Bushmaster with a 1:9 twist, and the heaviest bullet it will handle with acceptable accuracy is 62 grainers.

I'm shopping around for a 1:8 or even a 1:7 upper too.

If you want to be sure you can use 75-77 grain bullets buy a 1:7. I learned this the hard way.

quote:

Originally posted by Ammoloader:
If you want to be sure you can use 75-77 grain bullets buy a 1:7. I learned this the hard way.

I agree with you up to a point: That's what the theory says, and I believe the theory is — generally — correct more often than not.

But one can never be absolutely sure about a specific rig without having tested it.

If you want to be sure, you need to trial-and-error your choices in your own rifle to determine whether or not the theory applies to it.

This a question that can only be answered empirically. Or so I'd argue.

quote:

Originally posted by B L O'Connor:
The only way you'll know is by actually shooting different bullets.

If I understand it correctly, the theory is that a fast twist barrel will shoot heavier/longer bullets more accurately than a slower twist barrel would shoot the same bullets.

And vice versa: according to the theory, a slower twist barrel should shoot lighter/shorter bullets better than a faster twist barrel would shoot the same bullet.

I have a 6mm (1:12) which keyholes bullets that are too long/heavy (spitzers above 90 grains). It lives by the rule.

On the other hand, I have a .223 (1:9) that is sub-MOA with short/light bullets (50 grain). It does not live by the rule."

I don't understand your point. I would expect a 1 in 9" twist .223 to shoot light bullets well. What is considered a fast or slow barrel twist varies by caliber. A 1 in 9" .223 barrel is rather slow while a 1 in 9" .308 barrel is quite fast.

The Greenhill formula tells you that you need tighter twists for longer bullets, but does not say that shorter, lighter bullets will not perform well in a tight twist.

Many folks are shooting 40-55 gr. bullets from a 1:8 twist .223 barrel with excellent accuracy. Each barrel may like a particular bullet a bit better than another, but the "rule" is far from empirical.

Clarence

It all depends on what you want to shoot. The M16-A1 had a 1:12" twist and was designed to shoot 55gr military ammunition. The M16-A2 came out with a 1:7" twist and was designed to shoot 62gr military ammunition. The 62gr (M855) military ammunition is long for its weight because it contains a steel core and less lead. It is about as long as a 69gr match bullet. The 1:7" twist is marginally okay with the 55gr military bullets but the 1:12" twist does poorly with 62gr military bullets. Again, this was based around military ammunition.

Civilian rifles started being offered in 1:9" twist so they could do a decent job with both 55gr and 62gr ammunition. Today, most match shooters are using heavier civilian ammunition with 69gr-77gr bullets that require a 1:7" twist. But match ammunition can be found in other weights, including some with 52gr bullets.

So, in general:
1:12" twist best for light bullets of 50gr to 55gr
1:9" twist best for bullets of 55gr to 62gr
1:8" twist best for bullets of 62gr to 69gr
1:7" twist best for bullets of 69gr to 77gr

Of all of those I consider 1:9" and 1:8" the best compromises for shooting the widest available selection of civilian and military ammunition. For longer ranges and matches the 1:7" is what people are looking for.

Each of those twists will do differently stabilizing bullets that are heavier or lighter than what I listed and that depends upon the individual bullet and rifle. A longer barreled rifle will produce a little more velocity and impart a little more rotational velocity to the bullet. So, all else being equal, a 20" rifle barrel will stabilize a heavier bullet than a 14" carbine barrel. Bullet selection can also make a difference. Longer (not heavier) bullets require more rotation (faster twist) to stabilize. For example, a 62gr non-lead bullet will be longer and will do better with a faster twist than a lead core 62gr bullet that is shorter.

Again, 1:8" is a good all-around choice.

I was hoping to shoot 40-50 grain bullets in this AR. Based on what you guys are saying, that might not produce very good accuracy.
I am going to pass on the gun I was looking at.

For that that you should be looking for a 1:12" twist. My daughter's mini-mauser is very, very accurate with 50gr bullets. It has a 1:12" twist (maybe 1:14"?).

I am shooting a Bushmaster Predator with an 8 twist right now. It will consistently put 10 shots in a nickel at 100 yards with 50 grain Ballistic tips. It will not, as of yet put 3 60 grain partitions in a silver dollar, but will put Noslers 64 grain bonded and Barnes 62 grain TSX easily into a quarter. My niece killed her first two deer this year with the 64 grain bonded performance. Complete pass throughs that looked just like holes from a 260 Rem.

The 1-8" twist has shot nearly every bullet from 40 to 80 grains into well under MOA for me, through many barrels.

I'm primarily a target shooter and I use mostly match bullets, and I moly 'em. The only bullets that didn't easily make that standard were 55 FMJs or 62 mil-spec.

In rough 1-7" chrome bores, I'll sometimes lose a thin jacketed bullet if a bore gets really fouled but that's the only twist-related condition I've found.

HTH

Mark

Here is a little information about by personal experience with a 1:7 twist AR.
I have a varmint model which I tried several different bullet weights in. I started with 50gr Nosler BT. The groups produced were disapointing. I then switched to a 55gr Nosler BT. The groups were not much better. I then switched to a 69gr Match bullet. I still did not have groups that I felt the rifle should be capable of.
I called the manufacturer & dicussed the issues I was having. I told them I had purchased some 75gr Hornady match ammo to try. They suggested that I go ahead & try it & if it didn't shot some decent groups then send it back & let them check it out.
The rifle woke up & started shooting like it should.
The sum of all this is that the fast twist in rifle did not like anything under 75gr.

I have RRA 1:8 twist. I have not found any bullet from 50 to 72 that wouldn't group sub MOA. Most are in the .5MOA even military surplus ammo.

In my opinion changing the twist won't make a bad barrel good. Or vice versa. Just because you THINK you are only going to shoot 50 gr don't lock in of a slow twist. Get a good quality barrel your groups won't go to heck simply because you buy a 1:8 vs 1:12.

I have an AR Performance barrel with a 1/8 twist and a Wylde chamber.
It shoots any bullet from 52-75 grains with excellent accuracy.

I have a 1-9 Savage that shoots everything .5 MOA or less, even 75 grain HPs. I can't imagine a 1-8 not being able to handle anything. Some rifles prefer different bullets, doesn't mean they aren't stabilizing them. JMO.

Stability depends on more than a few factors, but primarily bullet length and twist rate, followed by velocity (remember, it is RPM we are looking for).

Just because a barrel is advertised as a 1-in-9 or whatever does not mean that it is so, measure the twist yourself ( cleaning rod, tight patch, marker and ruler will do).

I have shot 75 hornady's from a 1-9 at 600 and scored my PR, I know it would not stabilize 80 Amaxs.

I shot a 1-8 Badger with everything from 52 to 80s and they all would hold sub minute....

I currently shoot a 1-7 and 52s punch bug holes from the bench with a scope, and 80s perform wonderfully as well.

Summing, measure the twist yourself, push bullets adequately and they will often shoot very well, even in shorter 16 and 20 inch AR barrels.

Don't expect a 1-9 16 inch to shoot 80s at all, and some 77/75s at mag length either if the math results in a slow RPM....

Some say that short light bullets don't shoot so well in fast twist barrels, yet I have not seen that really impact anything I have shot, espc. in 223 and 223/556 variant chambers.

Note: a true 223 rem chamber is not necessarily the ideal chamber for an AR, espc if shooting heavies ( 70+), as freebore is a key to keeping pressures down and velocities up, and even a bigger issue if trying to shoot VLD/80+ heavies from any barrel length....

A 1-9 18" or longer barrel will likely shoot anything up to 77/75 mag length loads with aplomb.