# New ar-15

#### budlight

##### Well-Known Member
SS 24 inch Match .223 fluted barrel 1:9 twist
6X25 power 56 mm objective
JARD 2 pound single stage adjustable trigger
Nickle boron full auto heavy BCG
Forged 7076 T-6 upper and lower

Total weight 12 pounds 9 oz with 10 shot clip

I just bought a 5.56 20 inch fluted melonized and a 18 inch ss fluted both 1:9 just waiting on the guards and rifle length gas tubes.

#### Rob96

##### Active Member
Why 1:9 rather than 1:8 or 1:7 so 75-77gr pills could be used?

Very nice rigs

#### budlight

##### Well-Known Member
Why 1:9 rather than 1:8 or 1:7 so 75-77gr pills could be used?

It is math. As velocity goes up the less barrel twist required. Look it up. Going from 16 inch to 20 inch is about 200 fps. going from 16 to 24 inch is @350 fps In general terms that I have verified on extensive chrono time. So if you have a short 16 go for the 1:8 or 1:7.

I also have a 1:7 20 inch and it shines with 75 -80 gr. match bullets, but not as well as my match grade 24 inch 1:9.

So once you go up in barrel length forget the junk fast twist. Especially when you go to reloading near the 3300 fps bulk cheap 55 gr. I'm not bench rest shooting for a dime size hole in competition at 200 yards at @ 20 cents per bullet. I shoot the \$82.50 per 1000.

#### budlight

##### Well-Known Member
I actually found an on line calculator to figure out bullet rpm. This kind of explains the science of why you want less twist as your barrels get longer and the bullets increase in velocity.

Calculating Bullet RPM from MV and Twist Rate
The lesson here is that you want to use the optimal RPM for each bullet type. So how do you calculate that? Bullet RPM is a function of two factors, barrel twist rate and velocity through the bore. With a given rifling twist rate, the quicker the bullet passes through the rifling, the faster it will be spinning when it leaves the muzzle. To a certain extent, then, if you speed up the bullet, you can use a slower twist rate, and still end up with enough RPM to stabilize the bullet. But you have to know how to calculate RPM so you can maintain sufficient revs.
Bullet RPM Formula
Here is a simple formula for calculating bullet RPM:
MV x (12/twist rate in inches) x 60 = Bullet RPM
Quick Version: MV X 720/Twist Rate = RPM
Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.
Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.
Implications for Gun Builders and Reloaders
Calculating the RPM based on twist rate and MV gives us some very important information. Number one, we can tailor the load to decrease velocity just enough to avoid the bullet key holing the paper because of wobble or in extreme cases jacket failure and bullet blow-up at excessive RPMs. Number two, it helps us calculate the minimum twist rate needed to stabilize a particular bullet. Although there are other important factors to consider, if you speed up the bullet (i.e. increase MV), you MAY be able to run a slower twist-rate barrel, so long as you maintain the requisite RPM for stabilization. (Note: this is a general principle, but other factors, including bullet shapes, Center of Gravity (CG) and jacket thickness can affect stability). The slower twist-rate barrel may, potentially, be more accurate. Faster twist rates also cause increased pressure do to friction and less velocity
That said, we note that bullet-makers provide a recommended twist rate for their bullets. This is the “safe bet” to achieve stabilization with that bullet, and it may also

#### Kansaswoodguy

##### Well-Known Member
Nice guns makes me want to build another myself

#### budlight

##### Well-Known Member
I just took this new one out for a barrel breakin. All I had was a 12 inch utg pro hand guard. I ordered a ultra light free floating 9 inch

18 inch SS Wylde 1:9
Mid length gas tube
Anderson forged 7075 T6 aluminum upper and lower.
nickel boron bolt

7

#### 704

##### Guest
how did the 1/9 work for you?
I have always had better luck with 1/8 when running 62 gr and up

#### 300whisper

##### Well-Known Member
LRH Team Member
how did the 1/9 work for you?
I have always had better luck with 1/8 when running 62 gr and up

I'm having my S&W MP re barreled to a 1:7, stainless steel, 20 inch shilen. He said he wouldn't do anything other than a 1:7 twist. Budlight, your post about twist is interesting and informative. My buddy has the same barrel installed by the same smith but did a 1:8 inch twist. I think he was onto the right idea but did not have the science behind it to back his claim.

#### budlight

##### Well-Known Member
I'm having my S&W MP re barreled to a 1:7, stainless steel, 20 inch shilen. He said he wouldn't do anything other than a 1:7 twist. Budlight, your post about twist is interesting and informative. My buddy has the same barrel installed by the same smith but did a 1:8 inch twist. I think he was onto the right idea but did not have the science behind it to back his claim.

My first AR build was a learning experience 5.56 NATO 20 inch 1:7 and 55 gr. would fly at odd angles through paper at 100 yards. So I played with dumbing down the velocity to as slow as typical 16 inch barrels. I gave up and just went to 75 grain match bullets.

Any body want to buy the complete upper? Forged, full auto bolt, UTCpro handgaurd?

To answer the other ? my 24 inch match barrel is my most accurate rifle

#### budlight

##### Well-Known Member
how did the 1/9 work for you?
I have always had better luck with 1/8 when running 62 gr and up

With velocity less twist is required.. So in less than 18 inch a 1/8 would be fine. My 18 and 20 inch work well with 62 - 70 ish gr. My 24 Works very well with 75 grain match. I have not tried anything larger.

I didn't buy my barrels for competition shooting. I bought them for personal fun and critter getters. Last week in 4 days of ground squirrel hunting I thinned them out with 1300 some rounds. I got tired of looking through scopes all day long