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

By: Jose Chong

Jose Chong — Wed, 27 Mar 2013 17:18:16 +0000

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

By: Neil

Neil — Tue, 26 Mar 2013 21:22:02 +0000

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

By: Dasher

Dasher — Tue, 26 Mar 2013 15:07:47 +0000

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.

By: Norm

Norm — Fri, 11 May 2012 08:32:11 +0000

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

By: Neil Gibson

Neil Gibson — Tue, 08 May 2012 13:28:45 +0000

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.

By: Norm

Norm — Sun, 06 May 2012 13:39:02 +0000

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!

By: Koos Geldenhuys

Koos Geldenhuys — Sat, 30 Apr 2011 17:45:21 +0000

Why is that?

Because you saw something far better than
“Sinclair’s Simple Twist Rate Measurement Method” ????????

Don’t be naive!!!!!!!!

By: Koos Geldenhuys

Koos Geldenhuys — Sat, 30 Apr 2011 11:56:40 +0000

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.