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View Full Version : Finally buying an ACR! but... 1:7 or 1:9 twist?


RAP66
07-01-2012, 6:32 AM
I've been wanting an ACR ever since I found out they can be purchased in CA (state compliant of course) my few hiccups: I'm out of the states right now but will be traveling home next weekend :D and this junk with the senator :mad: I'm going to place my order now.

My question is barrel twist!?

Which do you suggest as a better all around choice? 1:7 or 1:9

I've read some of the forums and generally the opinion is mixed. Some say 1:7 is better vs the other, others say it depends on the weight of the bullet etc etc etc.

For the average joe, what do you suggest? which will give the better overall performance? I'll mainly be using this one at the range, not sure if I would take this one out on a hunt.

Plisk
07-01-2012, 7:07 AM
Chances are you going to use primarily 55gr ammo to go plink at the range.

Unless you're going to run a lot of heavier stuff (62gr+) you'll find 1:9 a good choice.

BucDan
07-01-2012, 7:11 AM
Average Joe... 1:9 twist as you'd be shooting 55gr rounds the majority of the time. With 1:9, it exists for the lighter rounds so it correctly stabilizes the bullet not "over stabilizing" which can affect accuracy and maybe precision.

Unless you have the "I want what the military has" ego and still not shoot 69gr rounds but "proud" to say you have 1:7.

TNP'R
07-01-2012, 7:12 AM
Chances are you going to use primarily 55gr ammo to go plink at the range.

Unless you're going to run a lot of heavier stuff (62gr+) you'll find 1:9 a good choice.

62 grain is ok in 1:9, anything above that is where a better twist rate comes into play.

pennys dad
07-01-2012, 7:21 AM
1:9 is standard but if you intend to use the longer rounds like 69 grn and up go 1:7

Richard Erichsen
07-01-2012, 7:32 AM
I've been wanting an ACR ever since I found out they can be purchased in CA (state compliant of course) my few hiccups: I'm out of the states right now but will be traveling home next weekend :D and this junk with the senator :mad: I'm going to place my order now.

My question is barrel twist!?

Which do you suggest as a better all around choice? 1:7 or 1:9

I've read some of the forums and generally the opinion is mixed. Some say 1:7 is better vs the other, others say it depends on the weight of the bullet etc etc etc.

For the average joe, what do you suggest? which will give the better overall performance? I'll mainly be using this one at the range, not sure if I would take this one out on a hunt.

Plenty of voodoo on this topic, but there is a more scientific approach. The Greenhill equation will get you close based on the loads you are actually planning to use. For example, let's say you want to use both M193 and M855 or loads similar to these. You'll need the following information to calculate the needed twist.

1) The length (note "weight" is covered by specific gravity)
2) The width
3) Muzzle velocity
4) Specific gravity (lead, steel, all-copper, etc.)

With these values, use this:

http://kwk.us/twist.html

Where to get military load information:
http://www.inetres.com/gp/military/infantry/rifle/556mm_ammo.html

M193: .76" long, .224" width, 3250 FPS, lead core/copper jacket SG of 10.9
NOTE: While the bullet is 55 grain, the tracer (M196) is 55 grain and .91" long, requiring a faster spin to stabilize and the real reason for the faster 1:12" and 1:9" twist found in military rifle barrels.

M855: .906" long, .224" width, 3025 FPS, lead/steel/copper jacket SG of approx 9
NOTE while the bullet is 62 grain, the tracer load is 63 grain and 1.15" long, requiring the much faster spin to stabilize and the real reason for the faster 1:7" twist.

Punching these in, you get the following:

M193 requires 13.2" to reliably stabilize
M855 requires 9.7" to reliably stabilize

However:

M196 tracer 9.3" to reliably stabilize
M856 tracer 7.1" to reliably stabilize

Barnes TSX (70 grain) and Nosler Competition bullet (77 grain) are longer than the 77 grain Mk262 (Sierra Matchking OTM). The Barnes TSX is 1.037" long, is all copper, so you need to use 8.9 for the specific gravity. Look up the lengths for each bullet you are interested in and plug in the MV and you'll get what you are after. In summary, the ammunition you plan to use will be your guide, stop guessing.

If you don't plan to shoot a lot of tracers in your rifle, the slower twist will be slightly more accurate and not as hard on the jacket. The one exception is long and heavy all-copper bullets which require more spin if driven at lower than typical military load velocities. For examples, the Barnes TSX needs a spin of 7.6" at the unusually high (commercial loads aren't moving this fast) 2800 FPS. The Nosler, Hornady and Sierra can be stabilized (if marginally) at 1:9" driven to higher velocities than most commercial loads utilize.

If you haven't a clue what ammo you'll be using, heavier bullets are used for longer ranges (in excess of about 400 yards) and are preferred for hunting deer sized game. Cheaper ammo for plinking at the 100 yard range are the various M193 loads that don't need a faster spin and actually suffer a bit for being overspun (but not by very much, it's all relative). If shorter range target shooting is what you are after, the 1:7" won't make much as much sense as something with a somewhat slower twist.

R

RAP66
07-01-2012, 7:50 AM
Yes, "Lots of voodoo"!!

My main purpose is to have fun, hunting I'll use a 308. I have my eyes on an 308 AR build as well.

1:9 will suffice for my needs, if I use heavier rounds I could always compensate for it.

:patriot:

FMJBT
07-01-2012, 9:28 AM
Personally, I'd rather deal with the effects of an over stabilized bullet rather than an under stabilized bullet. Over spinning a light weight bullet will at worst cause some amount of spin drift. Lower quality bullets will do this more than higher quality bullets, as it's based on how well balanced or concentric the bullet is. In contrast, under stabilizing the bullet will cause major issues with accuracy using heavier bullets. The bullets can begin wobbling pretty early in their trajectory, like limp wristing a football. I've seen 69 and 75 grain bullets keyholing into targets as close as 25 yards out of a 1:9 twist. I would only go with the 1:9 twist on something like a varmint rifle, where maximum accuracy with lighter bullets is it's primary purpose. For general use, I'd strongly suggest going with the tighter 1:7 twist because it will stabilize a much broader range of bullets.

Mossy Man
07-01-2012, 9:35 AM
and here's the next question...

how do calgunners feel about 1:8 :eek:

451040
07-01-2012, 11:03 AM
1:7

pinoy916
07-01-2012, 11:22 AM
1:7

And i love ALL 1:8 TWIST!!!!!

MrPlink
07-01-2012, 1:04 PM
Since when do shruby ACRs come in 1:7?

Rorge Retson
07-01-2012, 1:19 PM
Neither. 1:8. :D

Quiet
07-01-2012, 1:21 PM
Since when do shruby ACRs come in 1:7?
AFAIK...
Bushmaster was showing them at the 2012 SHOT Show.
Complete 5.56x45mm 1/7" barrel assembly = $400-500 depending on barrel length.

ACRfan
07-01-2012, 3:12 PM
There is only one barrel available for the ACR.. Haha so good luck trying to find other twists hahahaha

ScottsBad
07-01-2012, 3:50 PM
1:9 is standard but if you intend to use the longer rounds like 69 grn and up go 1:7

Standard for .223 maybe, but most good manufacturers of duty grade 5.56 rifles use 1:7 standard. I've frequently seen target barrels in .223 with a 1:9 twist. You must mean that it is the standard barrel for the ACR, yup.

People choose the 1:7 NOT because it's "what the military uses", but because it gives you a lot of flexibility to use heavy bullets if you wish. Maybe a 1:8 would be better for all around, but they are not widely available.

FN SCAR 16s uses a 1:7 twist and I would buy the SCAR before the ACR ANY DAY OF THE WEEK (and I have). Its a better rifle IMO and not because of the twist rate, although it would be one of several reason not to buy an ACR.

I had a DPMS Sweet Sixteen bull barreled rifle I built for target shooting. Nice little barrel for the price. It was a .223 with a 1:9 twist and shot very well with 55 grain, but that was it.

LaRue has a 1:8 on its rifles and uses a Wylde chamber in stainless. This might be a good option if you want a stainless barrel (another discussion entirely) and a Wylde chamber (yet another discussion). These are said to be very accurate.

In stainless, I like the BCM complete upper in 16" 5.56 (maybe it has a Wylde chamber, I can't remember) with a mid-length gas system.

If you have a very specific purpose for the rifle, buy the rifle or barrel that fits your purpose, but generally a 1:7 is what I buy. I primarily shoot 62 grain (or heavier), but I also shoot M193 some. For my purposes It works well and allows me to shoot a heavier bullet longer distances if I want. I do not shoot target competition.

A 1:9 twist make NO SENSE in a tactical rifle like the ACR, IMO.

B!ngo
07-01-2012, 4:49 PM
I've got a 1:8 and think it's a good compromise.
But in answer to the OP's question, it does of course matter what he plans to shoot. To answer that question you need to:
Reload and have a good sense of what weight (or range of weights of) bullet you plan to use or;
Survey what ammo is typically for sale and what you are likely to purchase and shoot. What I see is very little that is less than 55 gr and quite a bit more than 55 gr. So I would err on going with the 1:7.
B

1:7

And i love ALL 1:8 TWIST!!!!!

Richard Erichsen
07-01-2012, 5:48 PM
A 1:9 twist make NO SENSE in a tactical rifle like the ACR, IMO.

The majority of 5.56 NATO rifles around the world have a 1:9 twist and use either SS109 or a derivative of it (62 grain, .906" long same as US M855). The best explanation for that may be they don't care as much about tracer stability. There is no standard issue 5.56 load in most countries that is longer or heavier than current 62 grain SS109/M855 type ball ammo, making the 1:9 completely adequate. A typical US modern tracer that is 1.15" long it's going to by a wobbly goblin with this twist, but tracers are best used with SAWs and not for civi uses anyway.

R

Richard Erichsen
07-01-2012, 5:52 PM
I've got a 1:8 and think it's a good compromise.
But in answer to the OP's question, it does of course matter what he plans to shoot. To answer that question you need to:
Reload and have a good sense of what weight (or range of weights of) bullet you plan to use or;
Survey what ammo is typically for sale and what you are likely to purchase and shoot. What I see is very little that is less than 55 gr and quite a bit more than 55 gr. So I would err on going with the 1:7.
B

It's not about weight, it's about bullet length.

R

phish
07-01-2012, 6:08 PM
It's not about weight, it's about bullet length.

R

you can lead a horse to water...

Richard Erichsen
07-01-2012, 9:31 PM
Right you are, standing down....

R

ScottsBad
07-10-2012, 8:29 AM
There is no standard issue 5.56 load in most countries that is longer or heavier than current 62 grain SS109/M855 type ball ammo, making the 1:9 completely adequate.
R

The operative word here is adequate.

It's not about weight, it's about bullet length.


Sorry, but it is BOTH the length and weight of the projectile. It takes more force to spin a bullet that posses more mass. A faster twist will apply more force to the projectile to spin and therefore stabilize it in flight. Further, two projectiles of equal size with different weight (mass) will reach optimal stabilization in barrels with slightly different twist ratios.

In a static system (same pressure, same type of rifling), I believe that it can be shown mathematically that weight is a greater determinant in the necessary twist rate if the chamber pressures are equal. Length might, instead, determine the mechanical advantage of a projectile to take advantage of the friction applied by the barrel rifling to a larger surface area. All depends at this point on the type of rifling.

I believe barrel length and projectile weight are the largest variables in determining the correct twist ratio for any given rifling profile.

If this is incorrect, please explain where I've gone astray. I know this is a highly complex system with many variables, but I think this is correct.

Cheers-----

Richard Erichsen
07-10-2012, 11:03 AM
The operative word here is adequate.

Adequate means enough. It does not mean marginal.

Sorry, but it is BOTH the length and weight of the projectile. You are using the wrong term, the equation has variables for specific gravity and length. Specific gravity is part of the Greenhill formula and far more precise than "weight." For example, lead weighs more than copper and two projectiles of the same weight (say, 62 grain) will be very different in length. The URL I provided earlier includes these variables and explains why in the details of the Greenhill formula.

If this is incorrect, please explain where I've gone astray. I know this is a highly complex system with many variables, but I think this is correct. Review the URL and read up on Greenhill's formula.

R

vta
07-10-2012, 11:29 AM
55gr does fine in a 1:7. It's the real light varmint round like 52s and 47s that will over stablize.

digdug74
07-10-2012, 5:30 PM
I think it takes like a minute to change the barrel assembly on that rifle anyway so does the twist really matter? Get one or the other twist to start with and if you're not happy, buy the other twist latter on and you can toggle between them depending on what grain ammo you have on hand.