## TECH TIP: How to Determine Your Barrel’s Actual Twist Rate

Sometimes you’ll get a barrel that doesn’t stabilize bullets the way you’d anticipate, based on the stated (or presumed) twist rate. A barrel might have 1:10″ stamped on the side but it is, in truth, a 1:10.5″ twist or even a 1:9.5″. Cut-rifled barrels, such as Kriegers and Bartleins, normally hold very true to the specified twist rate. With buttoned barrels, due to the nature of the rifling process, there’s a greater chance of a small variation in twist rate. And yes, factory barrels can be slightly out of spec as well.

Before you purchase a bunch of bullets and set off to develop loads it’s wise to determine the *true* twist rate of your new barrel. Sinclair International, in its Reloading Press Blog provides a simple procedure for determining the actual twist rate of your barrel. Read on to learn how….

**How Twist Rate Affects Bullet Stability**

Most of you know that the twist of the rifling in the barrel is what puts spin on the bullet. As a bullet is pushed down the barrel and compressed into the rifling, the bullet follows the path or twist of the rifling. The combination of velocity and bullet spin is what stabilizes the bullet. Finding the twist rate for your barrel will help you in selecting appropriate weight bullets for your firearm. Remember, the general rule is that the faster the twist rate for a given caliber, the longer the bullet (of that caliber) you will be able to stabilize. (Generally speaking, a longer bullet will also be a heavier bullet, but the bullet geometry dictates the needed twist rather than the weight *per se*.)

**Determining Barrel Twist Rate Empirically**

Twist rate is defined as the distance in inches of barrel that the rifling takes to make one complete revolution. An example would be a 1:10″ twist rate. A 1:10″ barrel has rifling that makes one complete revolution in 10 inches of barrel length. Rifle manufacturers usually publish twist rates for their standard rifle offerings and custom barrels are always ordered by caliber, contour, and twist rate. If you are having a custom barrel chambered you can ask the gunsmith to mark the barrel with the twist rate.

Erik Dahlberg illustration courtesy FireArmsID.com.

**Sinclair’s Simple Twist Rate Measurement Method**

If are unsure of the twist rate of the barrel, you can measure it yourself in a couple of minutes. You need a good cleaning rod with a rotating handle and a jag with a fairly tight fitting patch. Utilize a rod guide if you are accessing the barrel through the breech or a muzzle guide if you are going to come in from the muzzle end. *Make sure the rod rotates freely in the handle under load.* Start the patch into the barrel for a few inches and then stop. Put a piece of tape at the back of the rod by the handle (like a flag) or mark the rod in some way. Measure how much of the rod is still protruding from the rod guide. You can either measure from the rod guide or muzzle guide back to the flag or to a spot on the handle. Next, continue to push the rod in until the mark or tape flag has made one complete revolution. Re-measure the amount of rod that is left sticking out of the barrel. Use the same reference marks as you did on the first measurement. Next, subtract this measurement from the first measurement. This number is the twist rate. For example, if the rod has 24 inches remaining at the start and 16 inches remain after making one revolution, you have 8 inches of travel, thus a 1:8 twist barrel.

This rifling illustration was created by Danish graphic artist Erik Dahlberg. It is published here courtesy FireArmsID.com, an excellent website for forensic firearms examiners.

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Jags fitted with patches, etc. never determine the twist rate of a barrel correctly.

The manufacturer of the best bullets in the world, developed the a perfect and very easy to make litle tool to measure the twist of rifle barrels 100% correct each and every time.

Follow this link and see how easy it actually is: http://www.gsgroup.co.za/twistchecker.html

Regards,

Koos.

Why is that?

Because you saw something far better than

“Sinclair’s Simple Twist Rate Measurement Method” ????????

Don’t be naive!!!!!!!!

The article is not quite true.

The twist rate can be roughly the number of inches that it takes for the bullet to make one full rotation, but that is based on a .308 calibre barrel as standard. ie .308″ times Pi = about 1″ so this leads to the confusion. In reality the true twist rate is a measure of calibres!

As an example.

Look at a 105mm cannon with a 1 : 15 twist. It would have a full rotation in about 4880mm of barrel length. Not 15 inches!

Dear Norm, exactly how does 15 x 105 mm equal 4,880 mm, it should be 1575mm? So in an M68 with a 5,350 mm bore, that give 3.4 full rotations to traverse that bore. Anyway whether you measure it as turns per inch, calibers or the actual helical pitch (in degrees, radians, etc), that fact is you can work out the rotational rate in RPM, revs/sec (rps), angle/sec, etc if you know the initial muzzle velocity of the projectile. From the MV you are able to work out the time for the projectile to rotate once. If you take the reciprocal of this, it gives you the frequency in Hz, which is the same as the rotation rate, so: MV in meters per second / twist rate in meters = rotational rate in Hz (rps). For RPM times by 60. For the M68 firing the M393 HEP at 731.5 m/s, this gives the RPM as 464 rps, or 27,866 RPM.

Neil,

You need to multiply the calibre by Pi to get the rotational distance of one full turn then multiply be 15.

i.e. 105mm x Pi = 325mm

15 x 325 = 4880mm

ie 1 full turn in mm x 15 = 4880mm

Not many of us shoot 105mm canons. All the rifle barrel manufacturers specify twist in inches, not caliber * pi (why pi anyways???). All the barrels I have measured have come out this way.

6mm 1:8 = 8″ = 203.2mm

6mm 1:8 * pi = ±150mm (so actually 1:6″)

Maybe it holds water with artillery, not with small bore calibers.

Norm:

I didn’t realise I hadn’t replied to your comment.

I’m sorry, but the term 1 turn in xxx calibres, means just that. One rotation in xxx calibres. So for the 105 mm tank gun example.

1 turn in 15 calibres = 15 x 105 mm = 1575 mm

It think you are mistaking the linear path travelled over the helical and rotational.

Linear path travelled in one turn is 15 x 105 mm = 1575 mm

Rotational path (i.e. the circumference) = pi x 105 mm = 329.867 mm

Helical path = square root of [(linear path x linear path) + (rotational path x rotational path)] = 1609.17 mm

The rotational and linear path distances can also be used to calculate the rifling pitch.

Arc tan (rotational path / linear path) = 11.829 degrees

The easy way to think of it is to unwind the helix and treat it as a right-angled triangle.

Dasher:

I rarely worked on 105’s, mainly 120’s or 155’s

Neil

I think I will stick with what the barrel maker says.