AR-15 ? 1in7, 1in9 twist in a 20inch barrel

[sauerofkraut]

I going to build an AR-15 to match my terrain, flat wide open northwest Kansas.  I'm going to go with a 20inch barrel for more velocity,  range, and reliablity. I'm going to stock 62grain to 77grain bullets.  Is there a huge loss of accuracy using 77grain bullets in a 1:9 twist?  Its hard to find a 1:7, 20inch, and chrome lined barrel.  This will be my first AR so I do not feel comfortable with building one.

[mountainredneck2051]

you can't generally use a 77gn bullet in a 1:9 barrel, it will not stabilize and will tumble right out the barrel

and in a 1:7 you don't want to use a 55gn bullet because it won't stabilize either

is there any reason why you want such a long barrel?

[sauerofkraut]

I was told that the 20 inch barrel with a rifle length is more reliable then the middleweight gas system.  More velocity for more effective round.

[Kobalt]

I have been researching the ar, and I think you should check this out.

http://youtu.be/hgcpR2QYBSY

[Reaver]

you can't generally use a 77gn bullet in a 1:9 barrel, it will not stabilize and will tumble right out the barrel

and in a 1:7 you don't want to use a 55gn bullet because it won't stabilize either

is there any reason why you want such a long barrel?

Generally.
1/9 will stabilize almost every thing. I would have no qualms with sending a slightly unstable heavy bullet down range.
It's better than the latter of POSSIBLY ( I've seen it fuckers so get off my back ) shredding a 55 grain in a 1/7

If I Reaver where to build an AR it would be a 1/9 because with that twist rate you can safely launch any weight bullet in 223/5.56 and have 0 worries about ripping the jacket off a 55 or below and having a completely useless shotgun.

[thatGuy]

Hey Sauerofkraut,

The length of the gas system has more effect on the velocity than the length of the barrel. Think about it, the gas expands till it hits the gas block at which point it exerts itself on the action of the rifle.

If you want to make max power you are right to go with the longest gas system. If you are doing it because of reliablity issues then you are barking up the wrong tree. Well built ARs are more reliable than anyone wants to give them credit for.

As far as twist rate goes a 1:7 will stabilize a 55gr-77gn bullet. A 1:9 will not stablize a 77gr. bullet. your call.

[sauerofkraut]

thanks for the info.  My miltia unit is thinking of standardizing to the ar platform.  I need to get get familiar with the characteristics of the system

[Reaver]

thanks for the info.  My miltia unit is thinking of standardizing to the ar platform.  I need to get get familiar with the characteristics of the system

What a stupid idea IMO

Yes, I understand logistically it's a good choice, but if they are not issuing ammo. Who fucking cares what I bring to the table.

[Kobalt]

thanks for the info.  My miltia unit is thinking of standardizing to the ar platform.  I need to get get familiar with the characteristics of the system

What a stupid idea IMO

Yes, I understand logistically it's a good choice, but if they are not issuing ammo. Who fucking cares what I bring to the table.

You run out of ammo, Its all Gone. Where do you get ak rounds? Err body has ar's. No ammo for reaver. Nuff said.

[Reaver]

thanks for the info.  My miltia unit is thinking of standardizing to the ar platform.  I need to get get familiar with the characteristics of the system

What a stupid idea IMO

Yes, I understand logistically it's a good choice, but if they are not issuing ammo. Who fucking cares what I bring to the table.

You run out of ammo, Its all Gone. Where do you get ak rounds? Err body has ar's. No ammo for reaver. Nuff said.

 if it wasn't mandatory to get an AR others would have AK's

[mountainredneck2051]

If you want to make max power you are right to go with the longest gas system. If you are doing it because of reliablity issues then you are barking up the wrong tree. Well built ARs are more reliable than anyone wants to give them credit for.

As far as twist rate goes a 1:7 will stabilize a 55gr-77gn bullet. A 1:9 will not stablize a 77gr. bullet. your call.

1:7 is known for shredding a 55gn

which is like the most common round out there.......


seriously though, modern ar15's are about as reliable as anyone could want
infact they will out do ak's in every realistic situation i can think of........

[thatGuy]

I shoot 55gr. out of my 1:7" all the time and have never seen it come apart.

[mountainredneck2051]

I shoot 55gr. out of my 1:7" all the time and have never seen it come apart.

and my buddy has over 1k of 77gn smk's throught his 1:9

huh, this koolaid is startin to taste funny 

[Kentactic]

its pretty simple...if your interested in accuracy at distance with your loads then there is a range of stability that must be maintained... there is a program that will tell you if your barrel/bullet combination will be within those specs. below is a link to that program. i gaurantee some of the above mentioned combo's will not fall into the acceptable stability range for optimal accuracy. but like the old saying goes "you dont know, what you dont know".

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[Kentactic]

oh and the acceptable range is between 1.3 and 2.0 for ptimal performance. higher thatn 2.0 and its over stable and below 1.3 and its probably going to be something like a knuckle ball.

[Reaver]

I'm not saying that you are going to rip apart every bullet you fire. All I'm saying is. 
" I have seen it happen, I'm not going to risk it "

Just throwing that out there for conformation that I am NOT saying " You shoot 55 grain's out of a 1/7 you'll shred it "


Everyone trackin?

[thatGuy]

I ran the numbers though JBM and a 77gr though a 1:7" twist at 2600fps came in at 2.123
and the same numbers for a 1:9" twist 1.28

If question was "will a 1:9" stabilize a 77gr bullet?" The short answer is no it won't.

[goodnightChesty1775]

i beilve an AR in 20" and rilfe length would be the most reliable because thats how it was designed. and the whole 1:7 vs 1:9 i went 1:9 because of what reaver said, and 0351 said he saw it happen too...

[thatGuy]

Is now a good time for me to mention that my dad swears he has seen aliens?

[Reaver]

Is now a good time for me to mention that my dad swears he has seen aliens?

I never take a mans word for granted until he proves other wise.


may this be a good time to mention that notion is part of today's problems?

[thatGuy]

It is a great time to mention that.

One of the problem with the interwebz is that everyone is an expert (myself included) and that compounded with anticdotal evidence turns random bull into fact.

I would like to go on the record saying I have never seen a 55gr bullet blow off it's jacket and I shoot nothing but the cheapest of the cheap.

That said, a 1:9" will not stablize a 77gr bullet which was the question. It wasn't 'will a 1:7" pull the jacket off of a 55gr bullet?'

[Reaver]

That's perfectly suitable to say.

But if the jacket had not ever been ripped off. why would it even be of question?

You note of 1.9 not stabalizing a 77 has been noted, and due to me never fireing it. I will not question your word until I prove other wise. If that day ever comes.

[thatGuy]

Once again, just my personal experience, 77gr flies like a son of a bitch out of my 14.5" 1:7" LMT. My grouping really tightens up with 77gr.

That could be me sandbagging though.

Maybe an experiment is in order.. I wonder if anyone I know has a 1:9" barrel??

[NOLA556]

Once again, just my personal experience, 77gr flies like a son of a bitch out of my 14.5" 1:7" LMT. My grouping really tightens up with 77gr.

That could be me sandbagging though.

Maybe an experiment is in order.. I wonder if anyone I know has a 1:9" barrel??

if you were anywhere near me I'd be glad to lend my 1/9 for the experiment...

[Reaver]

I have a 1/12
Maybe some ballistics from that style is in order as well.