Twist rate for an AR.... ???

mdoffroad · May 10, 2012

Ok so i am looking at my first AR build and see all kindas of the uppers that i want (Dissipator) but most are 1:9 and only a few in the 1:7. Mist of my ammo will be 69 or 55 grain rounds. Whats the upper limits for the 1:9 twist barrels? Thanks

lx1x · May 10, 2012

http://www.6mmbr.com/223rem.html

twist rates.. midway down.

sailskidrive · May 10, 2012

mdoffroad said:
Ok so i am looking at my first AR build and see all kindas of the uppers that i want (Dissipator) but most are 1:9 and only a few in the 1:7. Mist of my ammo will be 69 or 55 grain rounds. Whats the upper limits for the 1:9 twist barrels? Thanks

The question is whether or not the advertised 1:9 is really 1:9 or is it closer to 1:8.5 or 1:9:5. One of my guns w/ a 1:9 will stabilize 75gr bullets just fine while another won't stabilize 69gr.

If you're only shooting 55gr and 62 gr bullets 1:9 will serve you just fine.

mdoffroad · May 10, 2012

At most 69 grain

annihilation-time · May 10, 2012

The length of your barrel matters too. A shorter barrel will require lighter rounds in order to obtain optimal accuracy. A longer barrel likes heavier bullets.

For the 1:7 twist, I would recommend: SBR = 55gr, M4 = 62gr, SPR = 75gr.

I'm a 1:7 twist guy, so I can't say much about 1:9 twist. However, barrel length matters when trying to achieve optimal performance.

mdoffroad · May 10, 2012

16" barrel is the only way the Dissy style comes that ive seen. Wish they did an 18" version though

rseymorejr · May 10, 2012

lx1x said:
http://www.6mmbr.com/223rem.html

twist rates.. midway down.

Lots of good info there. Thanks for posting :thumbsup:

lx1x · May 10, 2012

rseymorejr said:
Lots of good info there. Thanks for posting

yup.. when i started reloading.. i found that site.. great reference.. :thumbsup:

annihilation-time · May 10, 2012

mdoffroad said:
16" barrel is the only way the Dissy style comes that ive seen. Wish they did an 18" version though

I don't know how I missed the dissapator part. That sums up the kind of day I'm having.

Boom Boom · May 10, 2012

Generally speaking, 69gr at best for 1:9. Get the 1:7. You'll be glad you did, more so as time passes, especially if you like/prefer 69gr rounds.

mdoffroad · May 10, 2012

Well, I don't exactly HAVER a preference yet, as this is my first AR. My goal is to build it from the ground up. My paperwork just came back today for my lower, so tomorrow I need to go and pickit up and start building. The upper is what is killing me in making a decision. Right now, it seems to be leaning towards Palmetto State Armory:

Barrel length 16"
Hammer Forged Barrel
Machine Gun Steel
5.56 Nato Chamber
1 in 7" twist
Double-thick Chrome-lined bore and chamber
A2 profile
Mid length gas system
M4 feedramps
Forged upper
T-marks
Full auto bolt carrier
Carpenter 158 Bolt

http://palmettostatearmory.com/inde...sa-16-hammer-forged-midlength-dissipator.html

All-in-all this seems to be a great setup for me, especially being new to building the black guns. Is there a downside to the 1:7 when shooting a 55 grain round?

Boom Boom · May 10, 2012

That PSA setup is very nice.

1:9 tends to be ideal for 40gr to 65gr. 1:7 tends to be ideal for 55gr to 80gr. Outside those bullet weight ranges for a given twist rate, the bullet may tend to tumble. There is also 1:8. It's a matter of what bullet weight(s) you prefer.

BradMacc82 · May 10, 2012

Don't read too much into my experience, but with both of my 1:9's I've run 40gr to 75gr with acceptable accuracy - not tearing one ragged hole, but printing nicely.

The main issue I've come across with the heavy match rounds is having to load them one at a time, since they were too long for magazine duty.

The 1:7 Dissy from PSA should suit you just fine, 55's should prove to be no issue.

Bolts Rock · May 11, 2012

BradMacc82 said:
The main issue I've come across with the heavy match rounds is having to load them one at a time, since they were too long for magazine duty.

Either you have non-GI mags or you're trying to stuff 75 A-Maxes into them. Black Hills 75 BTHP's will fit GI mags as will handloaded Hornady 75 BTHP's. 75 A-Maxes will not fit into any mags.

lkenefic · May 11, 2012

You can shoot a bullet with a shorter ogive, 69 gr for short (200 yd) and mid (300 yd) distances and save the longer heavier bullets (75 gr+) for longer distances (600 yds) when you're likely to be shooting slow fire anyway.

BTW... thanks for the link... lots of useful info!

BradMacc82 · May 13, 2012

Bolts Rock said:
Either you have non-GI mags or you're trying to stuff 75 A-Maxes into them. Black Hills 75 BTHP's will fit GI mags as will handloaded Hornady 75 BTHP's. 75 A-Maxes will not fit into any mags.

It was the A-max rounds, found out the hard way - as I normally do.

tommygun4hire · May 15, 2012

1 in 7

i have a colt h bar and its 1 in 7 i can ding the steel 10x10 plate every shot with 55 grain to 80 grain at 300 yrds

Twist rate for an AR.... ???

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