1/9 vs 1/7 twist?
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jpwilly
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If it's anything like my 1-9 AR it will not stabilize them and therefore completely negate any sort of effectiveness at pretty much any range. Although I've heard some people say their 1-9 bbls will stabilize 75gr bullets I'm pretty sure a 1-8 or even 1-7.5 twist would be ideal.
kwelz
Member
Just get 1/7 Trust me you won't regret it.
Bartholomew Roberts
Member
Some 1:9 barrels will stabilize the 75gr and heavier loads, some will not. The longer the barrel and the warmer your local temperatures are, the better your chances of success.
In my 1:9 16" barrel that did not stabilize the 75gr, I was still able to achieve 4-5" groups with match ammo at 100yds. While that is far from spectacular, it would let me use the rifle for home defense with no real issues.
Having said that, if you know you want to use the 75gr ammo, you should go at least 1:8 just to be sure.
In my 1:9 16" barrel that did not stabilize the 75gr, I was still able to achieve 4-5" groups with match ammo at 100yds. While that is far from spectacular, it would let me use the rifle for home defense with no real issues.
Having said that, if you know you want to use the 75gr ammo, you should go at least 1:8 just to be sure.
Well I was just wondering because stabilization has to do with accuracy at range.
The 5.56 bullet's effectiveness has to do with the speed at which it hits the target.
So I'm thinking shooting a heavier bullet outa 1/9 barrel shouldn't have an effect at home defense ranges because even though it doesn't stabilize as well it doesn't effect the velocity so much that it significantly brings the speed below 2700.
Am I correct in my assumption?
The 5.56 bullet's effectiveness has to do with the speed at which it hits the target.
So I'm thinking shooting a heavier bullet outa 1/9 barrel shouldn't have an effect at home defense ranges because even though it doesn't stabilize as well it doesn't effect the velocity so much that it significantly brings the speed below 2700.
Am I correct in my assumption?
FlyinBryan
Member
out of curiousity,
why do all the dcm high power shooters use 1/8 if 1/7 would be better?
why do all the dcm high power shooters use 1/8 if 1/7 would be better?
FlyinBryan
Member
didnt know that.
it seems like all the dcm rifles ive seen offered by the different companies have all been 1/8.
it seems like all the dcm rifles ive seen offered by the different companies have all been 1/8.
RRAs are, but white oak rebarreled mine and put a 1/7 on it
too many people want to shoot the 80s and 90s (JLK, SMK) and you need 1/7 for that, if not 1/6.5. also, with more people moving up from 69g to 75/77g at the short lines, 1/7 still makes more sense
too many people want to shoot the 80s and 90s (JLK, SMK) and you need 1/7 for that, if not 1/6.5. also, with more people moving up from 69g to 75/77g at the short lines, 1/7 still makes more sense
Bartholomew Roberts
Member
75gr out of a 16" barrel won't even reach 2700fps - I typically get around 2585fps with mine. Luckily for me, the 2700fps figure for fragmentation is for M193 or M855 ammo. The 75/77 rounds that are popular can still fragment as slow as 2200fps due to different bullet construction.
MTMilitiaman
Member
I don't even know why 1:9 exists at all, let alone how it got to be so commercially successful. If you want to use 55 gr bullets or heavier, the 1:7 is probably better. If you want to use the lighter bullets, a 1:12 is fine. Going with 1:9 is counter-productive because anything 1:9 can do, 1:7 can do better.
cameron.personal
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What? Try shooting 45-50gr loads out if a 1 turn in 7" barrel....
Let's look at what Colt has to say about the 1 turn in 9" barrels, right from their wesite:
How about Bushmaster:
I think people get stupid about twist rates because the military used 1in7 due to the LONG tracer rounds people think it is the be all and end all on 5.56 twist rates. The reality is that the 1 in 9 actually allows the greatest versatility for ammo. I have a 1 in 9" Colt HBAR that is the best with the widest range of ammo and will shoot 45 to 52gr match loads extremely accurately (sub MOA) while my two 1 in 7 barrels don't like the weights below 55gr and groups jump from sub MOA to 1.5+.
My rifles:
1 in 9" 16" Colt HBAR everything from 40-69gr ammo shoots well
1 in 7" 16" Lothar Walther match 55gr-77gr ammo shoots well
1 in 7" 10.5" LMT 55gr-77gr ammo shoots well (but the shortie loses range due to velocity loss)
Due to the availability of bulk ammo weights, and the high cost of heavier (69+gr) match loads and I think the best twist rate for the one AR guy for range training and home defense with perhaps a red dot or irons is the 1 in 9".
Cameron
FlyinBryan
Member
i think this guy has it.
ive only had 1/8 and 1/9 twist rate rifles.
the 1/9 barrels have been better with the common ammo that i load, which is usually between 40 and 68 gr. it definitly prefers lighter bullets, and just loves the 52g hornady a-max.
to be fair though, i didnt handload when i had the 1/8 colt, so i really dont know what it could have done if id put the time in it like i have the 1/9 barrels i have now.
moa is very common with my 16" hbar and 52g bullets, and even a few 5 leaf clovers here and there.
it deteriorates quickly when i get into the 68-69g match bullets though, go figure, lol.
Runningman
Member
+1.... as someone who has been shooting ARs more than 25 years must say excellent post, thank you.
Own ARs with 1 in 7, 1 in 8, 1 in 9, 1 in 12 myself.
RockyMtnTactical
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The differences between 1/7 and 1/9 are exaggerated by most. That said, WHY WONT THEY JUST START MAKING 1/8 THE DAMN STANDARD FOR CIVILIAN RIFLES!!!!!?????!!!!???? 
Bartholomew Roberts
Member
You don't know what the hell you are talking about. There isn't a rifle made with a twist fast enough to stabilize a 5.56x45 bullet in tissue.
For a good explanation of the physics behind spitzer/boattail bullets of all calibers tumbling in human tissue, see this link:
http://anarchangel.blogspot.com/2007/02/terminal-tumbling.html
You're the second "moderator" on this site to actually START an argument in a thread. When do you boys start moderating and stop arguing?
Quoting David Crane: Defense Review
Now, you can go into boat tails that will change the yaw of the bullet when it hits tissue, which is what the soviets did with their round. However, as stated above I am generally correct. If you want to nit pick it would be more polite to do so without jumping down my throat like a common thug.
RockyMtnTactical
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How so? 1/8 is perfect for just about everything, unless you are using M856 tracers.
Roadwild17
Member
So shooting through a 1/7 isnt going to do anything "magical" like spin the round so fast it sheds the jacket?
Bartholomew Roberts
Member
I no longer moderate here (thus the lack of bold name and the "Emeritus"). You see, I got tired of the daily grind of gun store commandos spewing misinformation on an hourly basis and decided to do something more productive with my time than gently correct their errors in a way that wouldn't trample their delicate egos and uninformed views.
As far as trying to start an argument, you don't know what the hell you are talking about and neither does David Crane. If pointing that out starts an argument, then I guess I'll have to risk offending your sensitive nature. Since you apparently don't find the cold, hard science in the previous link enough of an authority for your liking, let me add some more on that subject and see if any of that takes:
"The importance of rate of twist in wounding is a frequent subject of what we politely call "ballistic myth." Any projectile that has a "center of pressure" forward of the center of gravity will tend to tumble. You can illustrate this to yourself by trying to balance a pencil on your fingertip. Spin, given to the projectile by barrel twist, puts a projectile into a state described as "gyroscopically stable." The projectile might be momentarily disturbed but will return to nose-forward flight quickly. To describe how stable a given projectile is we use the gyroscopic stability factor (Sg). Generally you want a factor of 1.3 or greater for rifle rounds. 1.5-2.0 is a generally accepted value for 5.56 rounds.
For M193 the following variables apply:
axial moment of inertia (A) = 11.82 gm/mm2
transverse moment of inertia (B) = 77.45 gm/mm2
mass (m) = 3.53 grams
reference diameter (d) = 5.69 mm
Using the gyroscopic stability formula: Sg = A2 p2 / (4 B Ma) and assuming sea level we use an air density of 1.2250 kg/m^3 and discover that this this projectile will need on the order of 236,000 rpm for good stability (Sg > 1.3).
At 3200 fps M193 is typically spun up to more like 256,000 (1:9" twist) to 330,000 rpm (1:7") so that Sg approaches 1.9 or 2.0. 1:12" rifles will spin rounds at around 192,000 rpm and 1:14" rifles around 165,000 rpm. You can see why 1:14" rifles might have had trouble stabilizing M193 rounds.
Clever math types will see that density of the medium traversed (air in this case) has a dramatic effect on the spin required to maintain the Sg (density being in the first term's divisor). This is why cold conditions tend to dip "barely stable" rounds below the stability threshold. Without doing too much calculus it will be seen that an increase of three orders of magnitude (1000) in this variable will be a dramatic one for spin requirements. To balance things spin must be increased to compensate.
Through human flesh (which varies from 980 - 1100 kg/m^3 or about 1000 times the density of air) something on the order of 95,000,000 - 100,000,000 rpm is required to stabilize a projectile at speed. Given these differences it will be seen that the difference between a 1:12 or 1:14" twist when it hits flesh and a projectile launched from a 1:9 or 1:7" weapon is so small as to be beyond measuring. But the game isn't over yet.
Gyroscopic stability of 2.0 or so is sufficient for a M193 projectile to recover from an upset quickly, return to nose-forward flight and not be over stabilized. To prevent the upset in the first place, particularly when a sudden and very extreme change in density (and therefore drag and pressure applied to the center of pressure) requires FAR more stability. To grant enough stability force to prevent the upset of a M193 projectile encountering a sudden 1000 fold increase in density a factor of as much as 10 to 50 times (speaking VERY conservatively) the required gyroscopic stability for a steady state flight through a medium of that density would be required. In other words, unless the projectile is spinning at nearly a BILLION rpm it is going to be upset by such a transition. Even at this rpm it is like to be upset somewhat.
In summary, and to take the most extreme case, a M193 projectile spinning at 350,000 rpm (from a 1:7" rifle) is going to upset in flesh (yaw) exactly as fast as one spinning at 150,000 rpm (from a 1:14" rifle). Claiming that twist rate has any impact on the speed of yaw and therefore terminal performance is just not in line with the laws of physics. Anyone making such a claim should either be carefully avoided or introduced gently to basic gyroscopic stability concepts. Often a calming substance like warm milk or Thorazine helps in the transition of such a subject." Source
Here is also a nice scientific, peer-reviewed article describing myths about wound ballistics for physicians if you would prefer something more reliable than David Crane - http://linkinghub.elsevier.com/retrieve/pii/S0022534705623087
You aren't generally correct - you aren't even half-assed correct. You are so far from correct you couldn't see it with the Hubble telescope. So I don't really see it as nit-picking to point out that the Earth is not flat nor is the Moon made of green cheese.
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