Barrel twist vs. bullet weight

[iratollah]

I've seen lots of varying opinions and general rules of thumb. Seeking feedback from folks with actual trigger time.

My 16" AR has a 1:9 twist, (I'd have preferred 1:8 but couldn't pass up the deal that was offered.) I have a substantial quantity of 75 grain bulk milspec bullets for reloading.

How will these heavier bullets shoot in the 1:9 twist?

[WarHawk-AVG]

Heavier bullets do not stabilize as well in the 1:9 twist

My brother has the 1:8 and it stabilizes his 75gr very well (1/4" group at 100 yards) (he says the 55grainers open up a bit but he just uses those to plink)

The faster the twist the heavier the bullets (at lower velocity and less RPM's going downrange) it will stabilize.

1/9 twist is better for lighter, shorter cartridges (62 grain and below).
1/7 twist is superior for heavier, longer cartridges (62 grain and above).

Found this

The M16 family of weapons were fitted with 1:7 twist barrels in the early 1980s in order to stabilize the new composite core M855 5.56x45mm NATO ammunition, which was being adopted, along with the M249 SAW, at that time for US forces. Existing M16A1 rifles used a 1:12 twist rate for the rifling in their barrels, which was acceptable for stabilizing the then current M193 55-gr lead cored FMJBT round. M855 bullets have a center of mass further back on the round and this makes them more susceptible to tumbling in flight without a fast spin rate provided by barrels with tighter twist rates. As such the current M16A1 could not fire an M855 round for more than 100 yards or so without the onset of tumbling. I’m not sure the reasons why a 1:7 twist rate in particular was selected; there has been some speculation that this twist rate is a bit excessive for the M855 round. It is also possible that it provides extra gyroscopic stability for the use of other types of composite cored ammunition, existing or planned, such as the military M995 armor piercing round, etc.

So why do commercial AR15 clone manufacturers use a 1:9 twist barrel in their products? With the incredible rise in popularity of the AR-15 among civilian gun owners in the 1990s and 2000s, particularly post 9/13/2004, there existed a need for their products to fire a wide variety of ammunition with good accuracy. Commercial .223 Remington ammunition runs a wide gamut of bullet types, lengths, masses and configurations, so AR gunmakers building complete general purpose AR clones needed a rifling twist rate that would fire a jacketed, lead cored .224 bullet between 45 grains and 75 grains well. For this reason, a 1:9 twist rate works well because it will not overspin lighter bullets but can provide enough gyroscopic stability for the heavier 65-75 grain commercial ammo and thus it is used in virtually all civilian ARs. I personally prefer the 1:8 twist ratio myself, as it produces a bit better accuracy with the heavier and longer bullets.

Some manufacturers (Rock River, etc.) do offer the option of different twist rates for their rifles and these can also be sometimes custom ordered form the manufacturer. Colt used to sell all of their ARs with 1:7 twist and still do offer that rifling for their LE6920 rifles, which are the closest a civilian can get to owning an actual M4 carbine. The bottom line is that 1:9 twist is perfectly acceptable for regular target shooting and self defense with an AR, I prefer the 1:8 and the 1:7 is the current Mil-Spec

Its a combination of bullet weight, and speed and rifle twist

It still comes down to:
1/9 twist is better for lighter, shorter cartridges (62 grain and below).
1/7 twist is superior for heavier, longer cartridges (62 grain and above).
1/8 twist is a compromise that is suited but not better or worse on either end for all ranges of bullets.

[The Annoyed Man]

We have two ARs in the family:

1. A Bushmaster Varminter with a 24" 1:9 bull barrel. It likes the 68 and 69 grain SMKs and will shoot well under sub MOA all day long with those. It does not like bullets that are lighter or heavier, with the groups opening up to 1" with the 75 and 77 grain SMKs and larger with lighter weight bullets.

2. A Rock River Arms Varminter with a 24" 1:8 Wilson bull barrel. It likes the 75 and 77 grain SMKs and will shoot .25" groups all day long with those. It shoots 68 and 69 SMKs to about .75", and lighter bullets and 1" and larger.

Similarly, we have two heavy barreled bolt rifles in .308 in the family. Both rifles have 1:10 twists, one in a 26" barrel, and the other in a 24". Both rifles prefer 175 grain SMKs and will shoot .5" groups quite readily. They will both shoot the the 168 grain SMK into .75" groups; and they both hate anything lighter than 165 grains. However, another, lighter barreled .308 we own which has a 1:12 twist will shoot about 1 MOA with 150 to 165 grain bullets.

The bottom line is that our particular AR best results look like this:

Code: Select all

Weight     1:9    1:8 
<68         -      -
68          ✓      -
69          ✓      -
75          -      ✓
77          -      ✓

Given that these numbers do not jibe with what WarHawk has posted above, I'm wondering if barrel length isn't just as important? Isn't it possible that the shorter 16" barrels aren't long enough to stabilize those bullet weights at the slower twists, but the longer barrels are (increasing velocity in the process)?

[KBCraig]

Just to get all pedantic, it's not the bullet weight that determines the optimal twist, it's the bullet length.

And in a given caliber, the heavier the bullet, the longer it is. The smaller the caliber, the more pronounced the difference is. (I'm too tired to do the solid geometry at the moment, but it's easy to demonstrate.)

[The Annoyed Man]

Just to get all pedantic, it's not the bullet weight that determines the optimal twist, it's the bullet length.

And in a given caliber, the heavier the bullet, the longer it is. The smaller the caliber, the more pronounced the difference is. (I'm too tired to do the solid geometry at the moment, but it's easy to demonstrate.)

That actually makes some sense.

So, would that explain why a slower twist (1:9) can stabilize a heavier bullet ( 69 grain .223) if the barrel is long enough, versus not being able to adequately stabilize it if the barrel is shorter?

[iratollah]

For this reason, a 1:9 twist rate works well because it will not overspin lighter bullets but can provide enough gyroscopic stability for the heavier 65-75 grain commercial ammo and thus it is used in virtually all civilian ARs.

This seems to indicate the 75s should be okay.

I understand that 1:9 twist actually varies and can be closer to either 1:8 or 1:10, depending on the manufacturer.

Looks like I need to take 20 rounds or so of each to the range and see what works best. I just didn't want to have to mess with changing my reloading dies any more than necessary.

[WildBill]

Just to get all pedantic, it's not the bullet weight that determines the optimal twist, it's the bullet length.

And in a given caliber, the heavier the bullet, the longer it is. The smaller the caliber, the more pronounced the difference is. (I'm too tired to do the solid geometry at the moment, but it's easy to demonstrate.)

Let's get more pedantic. Assuming identical velocities, would a bullet of the same length made from a more dense material be more or less stable?

[KBCraig]

Just to get all pedantic, it's not the bullet weight that determines the optimal twist, it's the bullet length.

And in a given caliber, the heavier the bullet, the longer it is. The smaller the caliber, the more pronounced the difference is. (I'm too tired to do the solid geometry at the moment, but it's easy to demonstrate.)

Let's get more pedantic. Assuming identical velocities, would a bullet of the same length made from a more dense material be more or less stable?

For the exact same shape, the answer is definitely "yes; it will be either more or less stable".

Since there is little chance of experimenting with small arms projectiles made from anything denser than lead, testing this theory would meaning optimizing for something lighter, then reproducing it in lead. There have been aluminum-core bullets with lead & copper jackets, but of course these are lighter than what the gun is designed for, so they should stabilize just fine.

Stability is a function of the momentum of the rotating mass, just like a gyroscope. Since bullets are restricted to a given diameter, making them heavier means making them longer. Just like a gyroscope that is long and thin instead of short and squat, it's easier to deflect the long one if they're turning the same RPM. So to stabilize it, you have to increase the rotation speed.

Understand that this is conjecture, based on my understanding of the physics involved. In exterior ballistics, theories have a funny way of looking silly when tested in the real world.

If anyone wants to experiment with bullets made of iridium, platinum, gold or tungsten, I'd love to see the results. And the size of their bank account!

[WildBill]

Stability is a function of the momentum of the rotating mass, just like a gyroscope. Since bullets are restricted to a given diameter, making them heavier means making them longer. Just like a gyroscope that is long and thin instead of short and squat, it's easier to deflect the long one if they're turning the same RPM. So to stabilize it, you have to increase the rotation speed.

So, I would conclude that it's the bullet weight, not the length that determines the optimal twist.

[iratollah]

Turns out it's the length that determines optimal twist rate, weight is related to length of course.

I got this interesting Twist Rate Chart http://le.atk.com/pdf/223_MinimumTwistRates.pdf off another gun owners board I frequent. (http://www.2aforum.com" onclick="window.open(this.href);return false;).

Note that the recommended twist is based on <2800 fps loads. I think the best course is still to take my rifle to the range with 55s, 62s and 75s and see what I get. Armalite specs claim 2-2.5 MOA accuracy for my M4, it's important to keep that in mind as well when determining which load shoots best. If I'm not getting dime sized groups like my daughter's match grade rifle delivers, I need to remember the different barrels.