1:9 vs 1:7 Twist Rate?

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[ARCHIVED THREAD] - 1:9 vs 1:7 Twist Rate?

Posted: 12/13/2005 6:20:13 PM EDT

I keep hearing that if you want to shoot the 75gr TAP ammo you should have a 1:7 twist? However I just saw a review of a 16" barreled Bushy with a 1:9 twist where the best groups came with the 75gr TAP round. I own a Colt CNM 20" 1:9 twist and a Mini-14 by Accuracy Systems with a 18" barrel and 1:9 twist . I want to keep them both loaded with the 75gr TAP round as it appears to be the best performing round for .223 in terms of terminal performance. Doesn't barrel length play a part in bullet stablization as well as twist rate? Anyone else shot this round through thier 1:9 twist barrels?

Posted: 12/13/2005 6:28:45 PM EDT

[#1]

Some 1/9 barrels, especially the longer ones, will work with the heavy bullets. The only way to see if yours is one of them is to shoot it. Try to shoot it in the coldest weather you have locally, just to be sure.

Posted: 12/13/2005 6:32:20 PM EDT

[#2]

barrel length is a consideration only because of the increased velocity...

Posted: 12/14/2005 6:03:28 PM EDT

[#3]

Check this out.

www.ammo-oracle.com/body.htm#whattwist

Posted: 12/14/2005 6:20:48 PM EDT

[#4]

Quoted:
Some 1/9 barrels, especially the longer ones, will work with the heavy bullets. The only way to see if yours is one of them is to shoot it. Try to shoot it in the coldest weather you have locally, just to be sure.

How does the temp. effect bullet performance?

4X4SC

Posted: 12/14/2005 6:38:00 PM EDT

[#5]

1:9,1:7 I cured it by getting both

Posted: 12/14/2005 6:55:01 PM EDT

[#6]

www.ar15.com/forums/topic.html?b=3&f=118&t=260788

Posted: 12/14/2005 7:20:10 PM EDT

[#7]

Quoted:
Quoted:
Some 1/9 barrels, especially the longer ones, will work with the heavy bullets. The only way to see if yours is one of them is to shoot it. Try to shoot it in the coldest weather you have locally, just to be sure.

How does the temp. effect bullet performance?

4X4SC

Colder, denser air requires more RPM on the bullets part, in order to fly straight and not keyhole. When the military fired the original AR15 in 0* Fahrenheit and below temps, in testing, they found that the 55gr bullet was not stabilized by the 1/14 rifling, and they changed the spec to 1/12.

Posted: 12/14/2005 8:13:33 PM EDT

[#8]

Quoted:
Quoted:
Quoted:
Some 1/9 barrels, especially the longer ones, will work with the heavy bullets. The only way to see if yours is one of them is to shoot it. Try to shoot it in the coldest weather you have locally, just to be sure.

How does the temp. effect bullet performance?

4X4SC

Colder, denser air requires more RPM on the bullets part, in order to fly straight and not keyhole. When the military fired the original AR15 in 0* Fahrenheit and below temps, in testing, they found that the 55gr bullet was not stabilized by the 1/14 rifling, and they changed the spec to 1/12.

Interesting. Thanks for the explanation.

4X4SC

Posted: 12/14/2005 8:17:29 PM EDT

[#9]

My 16" 1:9 bbl will shoot about 75% keyholes with Hornady 75gr match ammo.

Posted: 12/27/2005 6:36:15 AM EDT

[#10]

I am searching for the best weight millsurp/reject I can find. So far it's Win ss109 and Radway Green ss109. 61.4gr and 60.6 gr respectively. Hope to get some 65gr, 68gr, and 70gr samples to try out as well.
I am shooting a 1:7 colt hbar. Avoiding the reloading bench for these. I may end up reloading my own in this as well.

Posted: 12/27/2005 6:37:51 AM EDT

[#11]

This topic comes up so often, we need a twist rate FAQ and have it tacked.....

Posted: 12/27/2005 7:35:16 AM EDT

[#12]

The 1/9 twist is about 50/50 with the longer, heavier bullets. Some will shoot them and some will not. Longer barrels seem to work better because the extra velocity helps.

The 1/7 is 100% sure to properly shoot(stabilize) the longer heavier bullets. Also with proper ammo selection you'll still be able to shoot ammo as light as 40gr.

If you currently have a 1/9 barrel give it a try and see what happens it may work and it may not but you won't know until you try.

If you are buying a new barrel, upper or rifle and want to shoot the heavier, longer ammo then get a 1/7 twist so you can be 100% sure.

Posted: 12/27/2005 8:29:18 AM EDT

[#13]

Quoted:
Check this out.

www.ammo-oracle.com/body.htm#whattwist

As I read this, AmmoOracle says to go with 1:7 only if you're primarily shooting the heavy stuff. Otherwise, their general recommendation is to go 1:9. Seems like 1:9 is a better choice if you're primarily a plinker.

Posted: 12/27/2005 8:40:19 AM EDT

[#14]

Don't forget the 1:8 option (5.56 chamber). I've had very good results shooting 69grn. Have not had the change to run 75 or 77 yet.

Posted: 12/27/2005 1:48:42 PM EDT

[#15]

Quoted:
This topic comes up so often, we need a twist rate FAQ and have it tacked.....

I second the motion

this is always coming up.

Posted: 12/29/2005 11:52:52 AM EDT

[#16]

Quoted:
Quoted:
Check this out.

www.ammo-oracle.com/body.htm#whattwist

As I read this, AmmoOracle says to go with 1:7 only if you're primarily shooting the heavy stuff. Otherwise, their general recommendation is to go 1:9. Seems like 1:9 is a better choice if you're primarily a plinker.

That's exactly how I read it as well. No one ever seems to talk about what (if anything) the 1:7 gives up when shooting the lighter bullets. There must be something - or else no one would manufacture anything BUT 1:7 twists.

Posted: 12/29/2005 2:00:45 PM EDT

[#17]

Quoted:
Quoted:
Quoted:
Check this out.

www.ammo-oracle.com/body.htm#whattwist

As I read this, AmmoOracle says to go with 1:7 only if you're primarily shooting the heavy stuff. Otherwise, their general recommendation is to go 1:9. Seems like 1:9 is a better choice if you're primarily a plinker.
That's exactly how I read it as well. No one ever seems to talk about what (if anything) the 1:7 gives up when shooting the lighter bullets. There must be something - or else no one would manufacture anything BUT 1:7 twists.

The 1/7 spins the lighter bullet faster than is ideal and in some cases if the bullet has a very thin jacket the bullet may come apart.

In theory since 1/7 twist overspins lighter bullets it will not shoot the lighter ammo as accurately as the 1/9 but most of us have found out the accuracy difference is not noticable especially if you are using a chrome lined barrel.

Some have also said that the 1/7 will have shorter barrel life than on with a 1/9 twist. When I pressed them for numbers I was told about 6% or roughly a 1000 rounds. IMHO, this is like the accuracy issue and is not even noticable. How the barrel is used will make a bigger difference in it's life span than than whether it's a 1/7 or a 1/9.

So to answer your question, the reason no one really talks about the the negatives of the 1/7 is because these negatives are also negligable for 99.9% of the users.

Posted: 12/29/2005 2:12:10 PM EDT

[#18]

Quoted:
1:9,1:7 I cured it by getting both

Me too!

Posted: 12/29/2005 2:38:13 PM EDT

[#19]

Quoted:
Quoted:
Quoted:
Some 1/9 barrels, especially the longer ones, will work with the heavy bullets. The only way to see if yours is one of them is to shoot it. Try to shoot it in the coldest weather you have locally, just to be sure.

How does the temp. effect bullet performance?

4X4SC

Colder, denser air requires more RPM on the bullets part, in order to fly straight and not keyhole. When the military fired the original AR15 in 0* Fahrenheit and below temps, in testing, they found that the 55gr bullet was not stabilized by the 1/14 rifling, and they changed the spec to 1/12.

Not only that, virtually all gunpowders lose energy when cold (some more than others), creating lower MV and compounding the problem because rotational velocity of the bullet is directly proportional to its linear velocity.

Posted: 12/29/2005 2:40:18 PM EDT

[#20]

Quoted:
1:9,1:7 I cured it by getting both

+1

Posted: 12/29/2005 2:43:14 PM EDT

[#21]

Quoted:
Quoted:
Quoted:
Check this out.

www.ammo-oracle.com/body.htm#whattwist

As I read this, AmmoOracle says to go with 1:7 only if you're primarily shooting the heavy stuff. Otherwise, their general recommendation is to go 1:9. Seems like 1:9 is a better choice if you're primarily a plinker.
That's exactly how I read it as well. No one ever seems to talk about what (if anything) the 1:7 gives up when shooting the lighter bullets. There must be something - or else no one would manufacture anything BUT 1:7 twists.

I am not a great shot but was shooting MOA or just under with a 1:7 CL 16" barrel using XM193

edit: it was about 20 degrees out

[ARCHIVED THREAD] - 1:9 vs 1:7 Twist Rate?

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