Precision measurement of barrel twist

Expanman · Jan 16, 2016

Does anybody know of a precision method of measuring rifling twist along the length of a barrel? I don't mean the method of marking a cleaning rod, fitting a patch tightly on a jag and measuring the distance the rod has to be inserted into the barrel to acheive one rotation.
I am interested in a method to check for a variance in the twist rate. A method to detect if the rifling in a 10 twist barrel goes to, say 5 twist or 0 twist for an inch or 2 then back to 10 twist due to a manufacturing error.
Any ideas?

GGaskill · Jan 16, 2016

A method to detect if the rifling in a 10 twist barrel goes to, say 5 twist or 0 twist for an inch or 2 then back to 10 twist due to a manufacturing error.

A gross error like that would be easily visible if you looked down the bore at a reasonably bright light source. Otherwise, you would need something like a lead lap (without lapping compound) that positively engages the rifling with a 360° protractor on the rod, an index for the protractor and an accurate yardstick for depth measurement. Then record the angle at half inch increments of depth and plot the results.

But an error of that magnitude would reveal itself by intense copper fouling at the site.

tobnpr · Jan 16, 2016

I would add that the large error you're talking about would be equally visible just watching the cleaning rod spin as you run it down the bore (all the way...) with the tight jag/patch.

Expanman · Jan 17, 2016

I had hoped the general conditions I presented would prevent hanging-up on the minutiae here, but it appears I failed. Allow me to re-pose the question. Suppose the twist rate varies from a 1-10 to a 1-9, and only for an inch or so, then back to a 1-10? I guess what I am asking is, does anyone know of an instrument capable of measuring degrees of rotation? I can conceptualize a method of measuring movement along the bore. What I am missing is a method to precisely measure how many degrees of rotation have occurred during that linear motion.

Expanman · Jan 17, 2016

Are you aware of a 360 degree protractor such as you mentioned that might be adapted to mounting at the end of a rifle barrel? I think you know what I'm looking for, I just have no idea where to look.

GGaskill said:
A method to detect if the rifling in a 10 twist barrel goes to, say 5 twist or 0 twist for an inch or 2 then back to 10 twist due to a manufacturing error.

A gross error like that would be easily visible if you looked down the bore at a reasonably bright light source. Otherwise, you would need something like a lead lap (without lapping compound) that positively engages the rifling with a 360° protractor on the rod, an index for the protractor and an accurate yardstick for depth measurement. Then record the angle at half inch increments of depth and plot the results.

But an error of that magnitude would reveal itself by intense copper fouling at the site.

SilveradoHauler · Jan 17, 2016

At one time Dan Lilja had photos of such a machine on his web site. Along with some history of the guy that made it. It showed deviation of twist along the length of the bore.

Google is your friend. Lilja Barrels.

Expanman · Jan 17, 2016

SilveradoHauler said:
At one time Dan Lilja had photos of such a machine on his web site. Along with some history of the guy that made it. It showed deviation of twist along the length of the bore.

Google is your friend. Lilja Barrels.

That is exactly what I was looking for. Thank you VERY much.

wesg · Jan 17, 2016

Expanman said:
Are you aware of a 360 degree protractor such as you mentioned that might be adapted to mounting at the end of a rifle barrel? I think you know what I'm looking for, I just have no idea where to look.

A 'degree wheel' for timing cams on engines.

JEGS Performance Products 81622, JEGS Precision Cam Degree Wheel | JEGS Performance Products

GGaskill · Jan 17, 2016

Try a school supplies store. This one is a set of thirty but you should be able to find a solo one somewhere.

Here is another and yet another.

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