What's the downside to 1:7 in an AR?

[dvdcrr]

The graph above is not quite correct for the heavier end of 1/9 bullets. The green should extend up to 69 gr and it's not correct to think that the twist becomes less than ideal as the bullet gets longer as long as it remains stabilized. As long as the bullet is stabilized the slower the twist the better for accuracy. A custom barrel maker will ask you for the bullet weight you are going to shoot and then recommend the ideal twist which is usually slower than you were expecting.

 

[joeschmoe]

Praire dog shooters like to use very light bullets. Faster twist needed for them. Otherwise 1/7-1/9 is GTG for most uses.

 

[SSN Vet]

My understanding was that the 1:7 twist could "over stabilize" the lighter projectiles, such that the axis of the bullet does not follow tangent to the curved trajectory path.

FWIW my 1:9 16" middy loves the 69 gr SMK

This explanation from over at shooter's forum is pretty good.....

The bullet needs to turn its nose into the air so that it continues to fly point first. This is often referred to as the bullet “going to sleep”. The aerodynamic shape will cause this unless it's over stabilized.*

If a bullet leaves the muzzle with a tiny bit of yaw (and they almost all do) and it's spinning too fast to aerodynamically correct its flight, you will lose accuracy and lower the ballistic coefficient as it's not presenting the most effecient frontal shape to the oncoming air.*

At longer ranges, generally past 300 yards, the bullet needs to turn nose down as the flight path begins to curve down so that it continues to fly point first. An over spun bullet will resist turning into the flight path and the nose will remain turned upward relative to the ballistic curve. Once again, the less efficient shape presented to the oncoming air will lower the ballistic coefficient making the bullet lose velocity faster and more prone to drift.*

You probably won’t notice an over stabilized bullet unless you’re shooting competitive benchrest or shooting out past 300 yards.*

It's not just length but also aerodynamic profile that determines how much spin a bullet needs to properly stabilize.

 

[SSN Vet]

.....

 

[Warp]

1:7 is the way to go unless you want to shoot very lightweight (like 40gr) varmint bullets.

The premium/defensive rounds typically use 75-77 gr bullets, which are great in 1:7. Economy 55gr are also very good in 1:7.

There area lot of less-expensive guns that use 1:9. Apparently it is less expensive to manufacture than 1:7...most AR buyers don't have a clue what twist rate they are buying or how it affects anything...and many (most?) only really shoot 55-62gr bullets most of the time anyway.

 

[Black Butte]

Praire dog shooters like to use very light bullets. Faster twist needed for them.

A fast twist is needed to stabilize heavy bullets. For lighter bullets, a slow twist is sufficient.

 

[Warp]

A fast twist is needed to stabilize heavy bullets. For lighter bullets, a slow twist is sufficient.

Correct. Other poster was confused and had it backwards.

 

[suzukisam]

you cannot over spin a 55 gr bullet with a 1:7. my WOA 1:7 shoots 1/2 MOA with 55 gr bullets.. 3-4 touching in a 5 shot group 100yds is not uncommon. I would never go slower than a 1:7.. you don't need to it just makes people with a 1:9 feel better to say you can over spin a 55 gr pill.... and at 300 yrds the 70 gr tsx is dead nuts on.. just ask the deer that keep falling dead

 

[Shurshot]

I just got this 1:8 barrel delivered today. I hope to be able to see for myself if it shoots any better than the 1:7 in my 6940. just need to gather up the rest of the parts I need... 1:8 barrels are harder to find.

 

[carbine85]

Those are nice barrels. Too much $ for my budget. I wished they offered a mid length in the M4 barrel. I might consider one at that point.

 

[Art Eatman]

FWIW, I have found that a 1:10 twist allows sub-MOA with 50- to 55-grain bullets. I also had sub-MOA with some round-nosed 70-grain bullets.

My varmint upper for my AR is 1:14, and I get one-MOA with 55-grain bullets and a Weaver K4 on it (for now).

 

[Lloyd Smale]

overspinning a bullet is much less of a problem then underspinning one. My 1-7 twist colt shoots 50s as well as my 1-9 twist ars do. Maybe if you were talking a 22250 or 220 swift where velocitys were much higher you could possibly overspin them but not in a 223.

 

[rskent]

Art
Where did you find a 1 in 14 barrel for an AR? I have been looking for a 1 in 12 for a while with no luck.

Sorry for the hijack.