Understanding Barrel Twist-Bullet Weights
Many times several of our FTF members and friends have had questions regarding barrel twist, ammunition selection and related questions. I had advised Dillinger some time back that sometime in the future I would do a thread to maybe help simplify it.
A general rule is the Heavier the Bullet you are shooting the Faster the Twist in the barrel should be. Of course, there is another extreme on the opposite side of the spectrum where the twist could be way too fast for lets say the lighter softer varmint bullet.
Barrel twist means the number of inches it takes for the lands and grooves otherwise known as the rifling to make a complete 360 degree rotation in the barrel. To simplify a 1:12 twist the rifling makes a complete 360 degree rotation in 12inches.
To explain the bullet being too light for the twist rate of the rifle. The bullet can literally come apart as it goes down range if the twist is too fast. This occurred when a very light varmint bullet was fired through a 1:7 twist barrel. At 200 yards not one particle of the bullet struck the target. The were coming apart in flight.
On the reverse, if you shoot 69-72 grain bullets in a 1:12 twist barrel as the M-16 A-1 Military Surplus Rifles some agencies have. The bullet will literally Key-Hole (side strike) when striking the target at 100 yards or less. This is due to the twist rate is not fast enough to stabilize the heavier bullet in flight. If you wish to do some math of your own for fun using the formula provided below you will find the effect of the twist rate in bullet rotation called revolutions per minute. rpm.
The formula sample below will be for a 1:12 twist rifle with the bullet velocity leaving the barrel of 3000 feet per second. The *Inches and the *Minutes are always constants in the formula.
Velocity 3000 fps X Inches 12 = 36000 Divided by Twist 12 = 3000 X Minutes 60 = 180,000 rpm.
* To figure the rpm for the 1:9 / 1:8 / 1:7 just pug the 9 -8 or 7 in to the twist rate position in the formula.
Using the above formula you will find that the 1:7 leaves the barrel at
So you can see the required increase in rpm that is required to stabilize a heavier bullet in flight.
Rifles are like people they have personalities. So the below is just a general chart as only a guideline of Barrel Twists compared to bullet weight. When working with a new round you should find ultimate accuracy somewhere in the bullet weight range below. I compare it to the weight (grains) of powder used when reloading. Usually the most accurate load or bullet lies somewhere in the middle?
Twist to Bullet Weight: Suggested Range
1:12 Twist 42-55 grain range
1:9 Twist 55-72 grain range
1:8 Twist 55-79 grain range
1:7 Twist 62-77 grain range
The above information is to be used for the basic understanding of Barrel Twist and Bullet Weight. There are always rifles that are the exception to the norm!
Determining the Twist of an Unknown Barrel:
To perform this you will need a Cleaning Rod with a Bearing Handle on it and also use a Jag with the Pointed Jag Tip, Silver Permanent Marker with a smaller point, a * Dry! Square 22 Cal Cleaning Patch and a ruler or tape measure.
It is best to take the Upper Receiver off of the Lower Receiver to perform this test.
With the Upper Receiver off now turn it upside down in a horizontal position and place in a vise to hold it steady with the Bolt Carrier Group removed. Position the Receiver so you are looking down into the bottom of the receiver at a 90 degree angle with the aluminum receiver surface just above the ramp area where the aluminum has been relived to continue the M-4 feed ramps. This aluminum ledge will act as a a reference point. (This reference point is the same one used when reading the throat erosion gauge on an AR) Then take the cleaning rod and install the cleaning patch on the Jag by inserting the Tip of the Jag in the upper left top corner of the *Dry Patch and rolling the patch onto the jag as tight as possible. Then insert the cleaning rod and *Dry patch in the rear of the Receiver keeping the patch into position. Start the patch into the chamber and bore approximately 4-5 inches. You will feel the cleaning rod turn slightly as it is inserted. Stop at this point! Look at 90 degrees down the receiver on the rod at the reference point and make a Silver Mark on the rod. Go back on the rear of the cleaning rod shaft about 5 inches from the handle and put a single Dot with the Silver Marker on the top of the rod.
Then begin smoothly inserting the rod holding your head in the same position watching the Dot until the Dot made at the handle makes a complete 360 degree revolution and comes into view in the same exact location in proximity to your head and eye as it started. *If the rod jumps while being inserted start over again! You will get a false reading if this occurs.
Once the mark has returned to the eyes exact view location after rotating 360 Degrees. Go back up to the Receiver reference point and make another Silver Mark on the rod at 90 degrees with the receiver reference point. You will then simply remove the rod and measure the distance between the first Silver Mark on the cleaning rod and the last Silver Mark you placed on the rod by the receiver. This will give you the Barrel Twist.
You can clean the rod marks off with solvent, gun scrubber or similar product.
can we get this stickied its good answers to very common questions
can you edit in a good easy description of the technique to determine the twist on an unknown barrel
Great stuff and certainly worthy of a stickie. One question...does barrel length have an impact on the bullets RPM at the muzzle?. For example I know that a 40gr varmint round will generally overspin and come apart with a 1:7 barrel but will a 16" barrel versus a 20" barrel make a difference?
Great post.. I had to read that section a few times to get it to all sink in when I read the book of the AR-15 by Sweeney.. I have now read 1 and 3 and are both a good read if you sift through the BS parts.
Got her done! Take a look at the first post.
Hope it makes sense!
I has sent JD a PM since I was not aware of how the Sticky works!
It will not effect the actual rpm out of the bore but will certainly effect the velocity. I just used the 3000 feet per second for the purpose of simplicity. Also there are a couple of ways to get the velocity figure for the formula. '
2. Contacting the manufacturers tech support and getting an average
velocity for the lot number run.
*I did add to the post a minute ago at the request of JonM regarding finding the barrel twist on a
barrel you do not know what the twist is.
You got it brother! Did that make sense to you! Sometimes I don't say what I am thinking correctly when it is in print!
ya it makes sense to me thanks
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