I hope I am not beating a dead horse.  I am STILL building my first AR.  I am trying to decide whether I need a 1 to 7 or 1 to 9 barrel.  I am primarily going to shoot cheap 55gr bullets but do plan on shooting some mid to upper 60's as well.  I am buying a 16in barrel and am wondering if I would have issues shooting some of the lighter 55gr bullets with the 1 in 7 twist?  If I can add any more info that will help you help me I can tell you anything you might need to know.

If all you plan to shoot is the "cheap" stuff, M193 and such and dabble with 69gr bullets, I'd go with the 1:9 twist.  I think it shoots trash ammo a bit better.  Depending on the actual twist of your barrel, 69gr will probably be the upper limit of heavy projectiles for the 1:9 barrel though.

Go with 1/8 and have some nice versatility.

1 in 7 does it all.

1:8 or 1:7

This

So you are saying that I won't have any issues running 55gr bullets in an 1/7 barrel?  I was under the newbie impression that key-holing was an issue with 55gr bullets.  Thanks for your help!

If you can't decide, split the difference and get a 1/8" twist barrel.

I would, but I am set on a Yankee Hill Machine fluted barrel.  They only come in the two twists.

I've struggled with this before, did some research, asked ARFCOM and came up with this:

The 1:7 helps stabilize the heavier military rounds and tracers. Varminters like 1:9 and higher which stabilize 55gr and smaller. The twist is not about bullet weight so much as bullet length based on: the heavier the bullet, the longer the length of the bullet, therefore the smaller the twist you need to stabilize the bullet; but not all modern 5.56/.223 in smaller weights are actually shorter.

I've read 1:9 is a good in between twist. If I had to bet, the 1:7 would be fine if you are not looking for MOA with 55gr and there are probably some 1:7 barrels out there that can shoot MOA with 55gr.

It really depends on what you want to do with the rifle. 1:7 will be fine as long as you are not shooting prairie dogs past 200yds-

1:7" shoots 55gr with zero problems.



 

Awesome, This is what I wanted to hear!  Thanks everyone's patience.

I disagree with that! I have a 1:7 and mine HATES 55 grain! Keyholes like a mother! Lightest I can shoot out of my 1:7 is 62 grain and it loves it. I have a 1:8 that is a dream. If I were you would go 1:8 that is perfect.

I had a 1/7 20" that is not good with 55gr.  The groups opened up by 2" with 55.   At the time, 1/7's were not common and it's nice to see them offered by most manufacturers.  If you have luck with 55 then great, if not, there are plenty of 62gr and above options available.

Specs????  Sounds like a rifle specific problem for you.

meh, I have a 1:9 rifle that shoots 75gr really good and has even stabilized some 77gr, I don't know what kind I borrowed some from somebody once just to see if it would stabilize them...it did, no keyholing but I didn't do any kind of accuracy test.  Sometimes I guess it just depends and you get lucky or unlucky.



To the OP, it's a compromise.  You can do 1:9 and have confidence that it will shoot 55gr good but might (and probably won't) stabilize anything heavier than 69gr.  Or you can go with 1:7 and realize it "might" not shoot 55gr as best it could but will shoot heavier than 69gr...like 77gr should you ever want to in the future.  Just depends on your priorities I guess.



 

1/7

Isn't 1/7 milspec?   Also isn't the normal milspec round (non ap) 55gr?

M855 isn't considered AP. Its the standard load.

 

I thought M193 was.  I hate being a newbie!

Have you read the ammo Oracle on this site? Download the pdf and read the whole thing (google: ammo oracle pdf)

Then be honest with your self about what you are going to shoot.  If the plan is to shoot 55 grain bullets then get a 1in 9" twist barrel.  The only reason not to is if you are planning to shoot heavier bullets exclusively.  As you stated before you are not.  The only way to really make an informed decision is to become informed before you make the decision.  Everyone on the internet has an opinion, and often these opinions are not based on any real knowlege or research.  Many of us have had good luck with 1:7, 1:8, 1:9, or even 1:12 barrels; that doesn't mean that the decisions we made were right or wrong.  My personal bet is that you will only shoot anything heavier than 62 grains a couple of times and all things being equal a 1in 9" twist will be optimum for you.  But please don't take my opinion read what the researchers have to say...

This is what the ammo oracle has to say:

Q. What twist rate do I want for my rifle?
Probably 1:9, but it depends on what kind of bullets you intend to shoot.
Special purpose rifles often have uncommon twist rates. For example, if you are
building a varmint rifle and want to shoot the short 35 grain, 40 grain, and 50 grain
bullets, a 1:12, or even 1:14 twist would be best. On the other hand, long range High
Power shooters often select 1:8, 1:7.7, 1:7, or 1:6.5-twist barrels to stabilize the long 77,
80 and even 90 grain bullets used for 1,000 yard competition. Additionally, new testing
of heavier rounds (68-77 grains) seems to show that they perform very well in simulated
tissue and may be a better defensive choice than 55 grain or 62 grain rounds. The
majority of shooters, though, typically shoot bullets of 50 to 69 grains in weight (note
that the 62gr SS-109/M855 bullet is as long as a 71 grain lead core bullet) and should
select 1:9 twist barrels. At typical .223 velocities, a 1:9 twist will stabilize bullet lengths
equivalent to lead-core bullets of 40 to 73 grains in weight.
1:12 twist rifles cannot stabilize SS-109/M855 bullets and 1:7 twist rifles are slightly less
accurate with lighter bullets and will often blow apart the thin jackets of lightweight
varmint bullets. The 1:7 twist is used by the military to stabilize the super-long L-
110/M856 tracer bullet out to 800 yards, but unless your plans include shooting a
significant amount of M856, the 1:9 twist rate is better suited for general use.
There is, of course, an exception: if you want to use loads utilizing the heavier, 75-77
grain match bullets currently used by Spec-Ops troops and other selected shooters, you'll
want a 1:7 twist barrel. Although military loadings using these bullets are expensive and
hard to get, some persistent folks have managed to obtain a supply, and will need the
proper barrel twist to use them. Anyone who foresees a need to shoot this ammo should
consider a 1:7 twist barrel.

Landcruiser Thank You!!!  After spending about a half an hour reading the Oracle's article I have decided on going with a 1/7 barrel.  It is my intention to shoot M193 the most, however since the cases are re-loadable I can roll my own heavier grain bullets.  Thanks for all who have helped with this process.

 

+1  I have two rifles with 1:7 and shot a ton of 55 grn through them.