1:8 twist rate

[Risasi]

Okay, I'm still shopping AR parts. Lowers are on the back burner for now, until I can find a decent supply. So I'm onto barrels;

I'm thinking of going with 18" barrels to give myself a mid-length gas system. I'm even sort of considering 20" barrels just so I could use a rifle length gas system. Likely I'll use CMMG.

I kept debating back and forth between 1:7 and 1:9, as I want to be able to shoot 55gr FMJ, yet at the same time some of the heavier 70+gr SP bullets. Likewise I also want decent accuracy. Well that got me to thinking, "why not just split the difference"? And that's when I started shopping 1:8 twist barrels. However I don't find a lot of info like gelatin tests, or reviews on odd duck barrels like this

So obviously I can shoot 55gr out of 1:8 barrels, but the real question is how WELL it shoots out of it? A; Is it accurate? B; Nothing crazy like "over stabilization" will prevent fragmentation of M193 FMJ will it?

Anybody here have 1:8 twist in 5.56?

 

[Coronach]

I've talked to a couple of people, via PM and phone about this very topic. The best way I've heard it described was this:

"1:8 will be just as inaccurate as 1:7 or 1:9. 55 gr ammo is 2MOA ammo, all day long."

I dunno. I figure a quality barrel 1:7 barrel shooting 55 gr ammo has the same chance of producing decent results as a rack grade 1:9 or 1:8 shooting the same stuff. Everyone seems to think the 55gr stuff is hit or miss...some barrels it likes, others it does not.

Most decent ammo (match grade or hunting) is heavier stuff (62gr on up), except for varmint loads.

Mike

 

[Jeff White]

There is nothing wrong with shooting 55 grain ammo out of a 1/7 twist barrel. It works just fine and is just as accurate. And no, despite what you might read on the internet, it won't overstabilize and no the jacket won't spin off. I shoot thousands of rounds of 55 grain through my 1/7 twist barrels every year.

You have to remember that 4 MOA meets milspec. The worst, most worn out M16 I ever took out of an arms room rack will most likely shoot 2 MOA or better. But Coronach is right, that's about the best you're going to get with 55 grain ammo.

Jeff

 

[DBR]

I just put together an AR15 upper. I used a CMMG 18" 1:8, rifle length gas system. My intended use is 55gr-77gr mag length ammo loaded to NATO spec. IMO the only reason to use 1:7 is for 80gr or greater target ammo - most of which will not fit in an AR15 mag.

The military adopted the 1:7 for special, long ammo (tracer) they fired.

Note: the twist rate is governed by the length of the bullet, not the weight.

If you want to shoot less than 55gr and light jacketed varmint type ammo then 1:9 is usually good for 45gr to 62gr. Individual barrels can vary a lot.

 

[Risasi]

...it won't overstabilize and no the jacket won't spin off...

Yeah, I wasn't worried about the jacket spinning off. Only certain light varmint rounds seem to have that problem. I meant overstabilize in such a way as to prevent fragmentation in soft tissue.

But I think that answers my question. Sounds like it doesn't really matter too much then. I'm not planning on shooting the real heavy grain stuff, as DBR pointed out much of it won't fit properly in a mag anyway. Likely I'll try to avoid 1:7's just because of barrel wear, but I won't sweat it too much. I guess whatever I find at a good deal, and is decent quality, is what I roll with when it comes time to buy barrels.

Thanks guys.

 

[Warbow]

I meant overstabilize in such a way as to prevent fragmentation in soft tissue.

No, that will never happen. The whole "slower twist rates will cause more damage in tissue because they're less stable" thing is a gun shop/forum myth.

 

[Bartholomew Roberts]

Yeah, I wasn't worried about the jacket spinning off. Only certain light varmint rounds seem to have that problem. I meant overstabilize in such a way as to prevent fragmentation in soft tissue.

I don't think that will be an issue. I've seen several M193 gel shots made with a Colt 1/7 barrel and they all show typical fragmentation for that round.

 

[mec]

The Bushmaster Predator has a 1/8 twist. Accuracy is not up to the level of my ruger .220 swift but groups of just over one MOA are pretty common. I get a lot of five shot spreads that put four in under an inch and one slightly out to mess up the average. My benchrest set up is fairly casual and about as effective as shooting from a harris bipod while sitting on the ground.

my notion is that this is a grab-it utility rifle. I don't care much about bench rest shooting or competition. Here are four separate 55 grain loads- one is my hornady sp handload plus a 69 grain sierra match bthp handload. The fake jr is 50 percent or less

 

[Risasi]

Thanks guys, this info helps a lot.


So I guess the next question in my mind is which barrel length to go with. I can't decide if I should go 18" or 20"...I'm a little hesitant to use 16" barrels as I don't think the carbine length gas system can be avoided.

 

[Bartholomew Roberts]

I can't decide if I should go 18" or 20"...I'm a little hesitant to use 16" barrels as I don't think the carbine length gas system can be avoided.

Midlength systems are fine on a 16" (I've got two that are perfectly reliable) and some people go as short as 14.5" In fact, the extra dwell time created by a 16" barrel and a carbine gas system is one of the things a midlength corrects nicely.

On an 18" barrel, you should be able to run a rifle length gas system reliably (which is what the military SPR uses on its 18" barrel). I wouldn't go any shorter than 17" with a rifle length gas system though.

 

[Risasi]

Okay, I have achieved resolution in my mind;


18" barrels, rifle length gas system, std front sight base, std handguards w/ attachment for light, 1:8" twist, 5.56 or wylde chamber.


18" because I want an edge over 16" carbine, however slight.


Rifle length gas system for obvious reasons.


I was very conflicted regarding the front sight base. And whether or not to free float the barrel. In the end I figured why spend more than $100, plus sights for a .2" edge in accuracy. This can easily add $200-$400 to each rifle. It seems especially frivolous when I can get pretty decent accuracy by picking a good quality barrel.


1:8" twist because I want to shoot 55gr-75gr without having to worry about hand picking barrels for accuracy. I believe as time goes on this will prove to be a solid twist rate for a general purpose rifle.


Now, I have to pick a brand. I'm leaning toward CMMG barrels. Anybody have an accurate brand of barrel they would recommend, that fits the criteria I just specified?



[EDIT] Edit to add; BTW I found this thread while searching the internet for an 18" 1:8 twist barrel. Google is really starting to weird me out, I keep finding my own posts.

 

[Bartholomew Roberts]

I was very conflicted regarding the front sight base. And whether or not to free float the barrel. In the end I figured why spend more than $100, plus sights for a .2" edge in accuracy.

Check out this thread for an excellent discussion of free-floated barrels v. non free-floated:
http://www.ar15.com/forums/topic.html?b=3&f=12&t=295633

That mirrors my own results when I first went to a free-float tube. Free-floated rails can also add other benefits like easy accessory attachment and better cooling too; though you can do just fine with the plastic handguards. It is like most things - you spend 50% more to get another 10% of performance.

On the 18" barrels, I hear a lot of good things about the WOA barrels and they appear to be a good value.

 

[DBR]

If you are going to assemble the upper yourself I would suggest:

CMT flat top upper
LMT mag particle bolt and carrier
You can get these from Bravo Company.

CMMG 18" 1:8 barrel with rifle length gas system and std front sight bridge unless you want optics only.
Smith enterprises Vortec flash suppressor (Phantom second choice).

If you are going to get assembled, just buy it from CMMG.

If you go with a free float hand guard I suggest the JP Rifles modular model.