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Posted: 7/19/2010 4:05:04 PM EST
I was wondering if somebody could help me out in understanding twist ratios? I understand that the twist is meant to stablilize the projectile, but what the big difference between 1.7 or 1.9? The BM ACR has the 1.9 and a lot of people dont like that, Why?
Link Posted: 7/19/2010 4:07:50 PM EST
1in7 twist stabilizes heavier bullets 75 grain and such, 1in9 can produce a keyhole effect with heavier bullets.
Link Posted: 7/19/2010 4:10:52 PM EST
[Last Edit: 7/19/2010 4:11:47 PM EST by FMJ]
Dont have a clue. unless you are a match target shooter

Ive shot 45gr to 77gr in my 1/9twist NO KEY HOLE so Im lucky
Ive shot 45gr to 77gr in my 1/7 twist

some like to shoot bullets over 75gr and want 1/7 or 1/8 ??? guess
Link Posted: 7/19/2010 4:20:31 PM EST
generally the faster twist is needed to spin the heavier projectiles faster to increase it's rang and accuracy. A lower twist ratio will not spin a heavier projectile fast enough to place it at it's best performance. So I have heard.
Link Posted: 7/19/2010 4:22:26 PM EST
Originally Posted By Jeepsnguns81:
1in7 twist stabilizes heavier bullets 75 grain and such, 1in9 can produce a keyhole effect with heavier bullets.


What exactly is a keyhole effect? DOes that mean that the bullet starts tumbling? Thanks for the help so far!
Link Posted: 7/19/2010 4:25:17 PM EST
Originally Posted By edogg_james4804:
Originally Posted By Jeepsnguns81:
1in7 twist stabilizes heavier bullets 75 grain and such, 1in9 can produce a keyhole effect with heavier bullets.


What exactly is a keyhole effect? DOes that mean that the bullet starts tumbling? Thanks for the help so far!


It means that the bullet is not sufficiently stable, and will exhibit yaw during flight. While it may not be tumbling, it will still print "keyholes" on the target.

Link Posted: 7/19/2010 4:26:19 PM EST
[Last Edit: 7/19/2010 4:27:20 PM EST by FMJ]
Originally Posted By edogg_james4804:
Originally Posted By Jeepsnguns81:
1in7 twist stabilizes heavier bullets 75 grain and such, 1in9 can produce a keyhole effect with heavier bullets.


bullet starts tumbling!




like shooting M855 in a 1/12 twist

Ive shot my 2 Bushmasters
A1 14.7 hbar 1/9
A1 20hbar 1/9
out to 200Y with no key hole useing 77grsmk



Still I own 1/7 twist M4`s


wish I kept the 20inch rifle

Link Posted: 7/19/2010 4:41:22 PM EST
This has been flogged beyond,,, again.
A basic knowledge can be gained by reading, something or anything about Ballistics. The Greenhill Formula, may help as well
458
Link Posted: 7/19/2010 4:54:59 PM EST
So what would be the ideal Twist ratio? 1.7?
Link Posted: 7/19/2010 5:47:30 PM EST
[Last Edit: 7/19/2010 5:52:57 PM EST by Peacemakerx]
check this What twist rate do I want for my rifle?

it really depends what you shoot 99% of the time, and what type of defensive ammo you may want to use, if you are trying to run 75 grain for defensive ammo some 1/9 barrels may like as referred above but some dont.

Link Posted: 7/19/2010 5:48:33 PM EST
I'll opine that 1 in 8" is overall best, but really you should read this:

twist rate faq
Link Posted: 7/19/2010 6:07:36 PM EST
Originally Posted By edogg_james4804:
Originally Posted By Jeepsnguns81:
1in7 twist stabilizes heavier bullets 75 grain and such, 1in9 can produce a keyhole effect with heavier bullets.


What exactly is a keyhole effect? DOes that mean that the bullet starts tumbling? Thanks for the help so far!


Keyhole Effect. This is 75 grain A-MAX loaded rounds fired from a 1/9" twist Stag FROM 25 YARDS

Found here: ARES DEFENSE GSR-35: gas piston retrofit range report. Page 4



Link Posted: 7/19/2010 6:27:46 PM EST
I have 2 1.9s. One loves anything I put in it. The other I won't shoot anything over 69 gr. Both shoot light weight varmit rounds great out to 200 yds and beyond.
My 1.7 loves 75+ but won't group 55gr ammo worth a damn.
I've come to believe every barrel is different!
Link Posted: 7/19/2010 7:03:06 PM EST
[Last Edit: 7/19/2010 7:03:48 PM EST by dlstarcher]
Overall, i like my 1:8 ratio better than the others,
i never had a problem shooting anything from 50gr. to 77gr. out to 300 yards. just my thoughts.
Link Posted: 7/19/2010 9:23:45 PM EST
[Last Edit: 7/19/2010 9:32:30 PM EST by js308]
Originally Posted By edogg_james4804:
Originally Posted By Jeepsnguns81:
1in7 twist stabilizes heavier bullets 75 grain and such, 1in9 can produce a keyhole effect with heavier bullets.


What exactly is a keyhole effect? DOes that mean that the bullet starts tumbling? Thanks for the help so far!


Think of throwing a football, a nice smooth spiral would be "stable", a throw with a lot of wobble would be "unstable"
Bullets do not tumble, they spin.
Link Posted: 7/19/2010 9:29:00 PM EST
[Last Edit: 7/19/2010 9:36:24 PM EST by js308]
Originally Posted By edogg_james4804:
I was wondering if somebody could help me out in understanding twist ratios? I understand that the twist is meant to stablilize the projectile, but what the big difference between 1.7 or 1.9? The BM ACR has the 1.9 and a lot of people dont like that, Why?


A twist rates indicates the distance the bullet must travel to complete one full revolution.
1:7 means the projectile will have to travel 7 inches to complete 1 full revolution.
1:9 meant the projectile will have to travel 9 inches to complete 1 full revolution.

Like the others said, the faster twist rates will spin the heavier grain (more modern) bullets faster to increase their distance.
Link Posted: 7/19/2010 9:29:31 PM EST
Higher twist rates are NOT needed for heavier bullets. They are needed longer bullets. It just so happens that the longer bullets are heavy, relatively speaking. Generally speaking, a run of the mill 1:9 twist barrel will stabilize 40 to 69gr bullets. A 1:7 will stabilize just about anything that doesn't evaporate when it leaves the barrel. That last comment was directed at light bullets with thin jackets.

Of course, there are going to be exceptions, but I believe that the above is right, generally speaking. I run a 1:8 twist barrel and it shoots everything that I've put through it accurately. So far, I've run 45gr all the way up to 77gr and it all shoots better than I can.
Link Posted: 7/19/2010 9:30:50 PM EST
[Last Edit: 7/19/2010 9:35:41 PM EST by js308]
Originally Posted By edogg_james4804:
So what would be the ideal Twist ratio? 1.7?


For the newer heavier grain ammo yes.
If your shooting 55 grain ammo, 1/9 would be just fine.
Link Posted: 7/20/2010 4:19:33 AM EST
By the numbers 1:7 is really too fast for anything that you would buy commercially.

I have 1:7 barrels that produce acceptable accuracy with 55gr bullets, but it is not the optimum twits rate mathematically.

As far as I understand it, the ARMY specified a 1:7 twist rate to ensure that the really long 69gr tracer rounds would be stabilized (even though they are only 69gr they are hollow in the rear to accommodate the phosphorus, thus the projectile is very long compared to a standard 69gr bullet and requires a faster rate of twist)

And because the ARMY wants it and it is called for in the TDP that means that it is the tacti-cool thing to have, even though it is probably not the optimum rate of twist for what 80% of us shoot.

I would suggest a 1:8 twist rate simply because that is what I have experienced the best results with… yes, numbers are numbers and by the numbers 1:8 is even a little fast for 55gr – 69gr, but my 1:8 barrels shoot great groups with 55gr-75gr projectiles and when it comes to firearms I think there is a little voodoo involved that defies the math sometimes.
Link Posted: 7/20/2010 4:50:20 AM EST
Or if you hand-load you can literally make any projectile work in any twist rate out to 200m.
For example:
In a 1/9 twist speed up the 68 grain and over bullets
In a 1/7 twist slow down the 53 grain and lower weight bullets.
I'm only talking about 6 inch groups out to 200m...basically minute of chest cavity out to effective quick shooting at defensive/assault distances
Link Posted: 7/20/2010 4:54:07 AM EST
Or if you hand-load you can literally make any projectile work in any twist rate out to 200m.
For example:
In a 1/9 twist speed up the 68 grain and over bullets
In a 1/7 twist slow down the 53 grain and lower weight bullets.
I'm only talking about 6 inch groups out to 200m...basically minute of chest cavity out to effective quick shooting at defensive/assault distances
Link Posted: 7/20/2010 5:33:24 AM EST
Oh, I forgot to mention this, but my carbine that I go humping around in the woods with and generally plink with is a 1:9. It shoots all of the run of the mill 55 and 62gr stuff just fine. I am running the 1:8 on my scoped rifle, but that's mostly because I built that rifle with the intention of running 69 and 77gr bullets through it. Personally, I've never been a big fan of the 1:7 twist barrels. I never shoot tracers and don't anticipate having to shoot 90gr bullets that have to be loaded one at a time.
Link Posted: 7/20/2010 5:44:28 AM EST
Originally Posted By edogg_james4804:
I was wondering if somebody could help me out in understanding twist ratios? I understand that the twist is meant to stablilize the projectile, but what the big difference between 1.7 or 1.9? The BM ACR has the 1.9 and a lot of people dont like that, Why?


because bushmaster has a habit of taking a good thing and fucking it all up.

a rifle at the price of the ACR should be on par with other premium 5.56/223 rifles as such i should be guaranteed to shoot 75/77gr projectiles such as Hornady TAP or Black Hills MK262.

the ACR was in development for what? 2-3 years? in that time, bushmaster couldn't setup their tooling to design a better barrel profile specifically for the ACR? instead they scooped up whatever M4 profile barrels that were laying around and used them for the ACR

i think more people were pissed that BM's response was "1/7 twist will only be available on the remington 'military' line of ACRs" and that they used a barrel profile specifically designed for mounting an M203 in a brand new rifle that has no provision for mounting such a weapon. it was bushmaster corner cutting at its finest with a pricetag that rivals some of the best AR style weapons on the market.

would you pay Nighthawk Talon prices for a Springfield GI?
Link Posted: 7/20/2010 6:06:27 AM EST
Both my RRA and Stag Arms are 1 in 9 twist barrels. They seem to like from 50 to 62 grain ammo just fine. That makes them a good choice for larger varmint such as coyote which I enjoy hunting. I have a 1 in 12 CZ 527 bolt action that uses the lighter bullets up to 50 grain and I use it to pop ground squirrel and the likes. If I were target shooting I would probably go with a 1 to 8, or 1 to 9 twist and use the heavier ammo. I don't target shoot therefore I find the slower twist rates to be perfect for what I do. The ammo lighter ammo is also easier to find at local shops in faraway places when hunting. Get whatever suits your needs and will give you the flexibility you are going to want. Target, or long range shooting only, heavier bullets. Popping varmint go with a lighter faster bullet. They will still get you out to 300 yards and kill efficiently. My RRA is for home defense and I'm fine with a 62 grain pill.
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