Test for barrel/bore straightness?

[OrangePwrx9]

I've noticed on my inlines that zero well and shoot normally, I can see a diffraction pattern of four or more bright concentric circular rings down the length of the barrel when I put a bright white LED light behind the nipple and look in from the muzzle.

I've got a new inline that shoots very low and won't zero within the rear sight's (considerable) range of adjustment. Last night after I cleaned it, I stuck the light behind the nipple. I see one bright white ring back near the breechplug, then a washed out yellowish ring outside the white ring that is NOT concentric and no more. In other words, the first bright ring is off to one side of the second ring which itself is lopsided.

Wondering if there are any machinists, testing experts, physicists or other members of the HR brain trust that could tell me whether the absence of the normal diffraction pattern says anything about the barrel's bore straightness.

Researched this a bit in Wikipedia and found the subjects titled "Fraunhofer pattern", "Fresnal diffraction", and "Airy disk" seemed to discuss the phenomena. The precondition was that the light must pass through a pinhole or slit that's comparable to the wavelength of the light. Light passing through a nipple aperture would meet this requirement.

 

[rcmodel]

Concentric rings of light is what manufactures used to use when straightening new barrels with a barrel press.

Savage at least still does.

See this about that:
BARREL-STRAIGHTENING MACHINE about 2/3 way down the page.
http://www.nps.gov/spar/historyculture/machines.htm

The guy using the barrel machine looked through the new barrels at the rings of light, and used the over-head hand wheel to bend the barrel straight so all the rings were concentric.

It sounds to me like your first barrel is straight, and your second barrel isn't.

rc

 

[ironworkerwill]

Id bet the rings tell the tale! we use this method to indicate if a carbon arrow is striaght. the only thing is- what if the nipple was not in the center of the bore? would this cause the rings not to be concentirc? I think if the flash hole is centered in the bore you might be able to see some droop in the barrel using a straight edge, that is if the barrel dosent have too much conture or not straight for enough distance at the breech area.

 

[Doak]

Another way, is to get a good view of the bore, either by removing the nipple or the breech plug, and looking thru the bore at a vertical, dark, straight line on a white background. The line will cast it's image on the bottom, top, or both surfaces, of the full length of the bore. If the line you see, on the surface(s) of the bore, is straight & unbroken, the bore is straight. If the line is separated, forming 2 or more off-set lines, the off-sets are where the bends are. You can rotate, & angle the bore, while looking thru it, for different effects.

 

[OrangePwrx9]

That's interesting, Doak. I might give that a try.

I swabbed some of the protective lube out of my barrel yesterday and took another look with the bright light through the nipple. I am seeing more than two rings, but they're not concentric and only the smallest/first one is circular. The rest are lopsided toward a specific side of the barrel.

It might get another try at the range, but it'll be headed back to its maker shortly. Range sessions are so few and far apart in the winter that I hate wasting them futzing with something that'll never work right.