View Full Version : Air gauging a barrel

January 27, 2000, 01:35 AM
I see where sometimes rifle barrels are advertised as being air gauged.How is this done? And what kind of equipment is used.I assume it is a more accurate way to measure the inside of the barrel.

January 27, 2000, 08:55 AM
.000010" (ten millionths of an inch)resolution capabilities. Very precise measurements can be taken.

A precision "handle" with air blowing through it is zero'd in a known size ring (say .038" inside diameter for a .308 cal bbl). The "handle" is then inserted through the bore & deviations from the ring's known zero (inside size of the ring) are viewed, on a dial, as + & - from that known size. Works off of back pressure of the air. The bigger the bore (relative to the zero) allows more air to pass (less back pressure) so shows up on the dial on the + side of the dial. Smaller bore diameter = higher back pressure = - side of the dial.

'Bout the extent of it. Above's a pretty simplistic explanation. If you really want more, I'll do it in more detail.

January 28, 2000, 01:41 AM
Thanks....sounds as though you have used one. Is an air gauge used only to measure cylinders? Or can it be used on any shape that needs to be measured. Also as a general rule are these used for small spaces only?

January 28, 2000, 08:08 AM
Labgrade, that's a great explanation. I am only familliar with airguages used for barrel measurements, however, rifle barrels aren't cylinders, they have internal geography to them.

Air guages for barrels must be made to the exact rifling twist and bore and groove dimensions for that caliber. Each airguage probe costs quite a bit of money and must be tested for accuracy from time to time to ensure consistency.

It's a pretty complicated system, really, but the actual testing is pretty straightforward. A standard unit with one probe can cost upwards of ten grand.

Hope this helps.

"We all have it coming, kid..." Clint Eastwood, Unforgiven

[This message has been edited by artech (edited January 28, 2000).]

January 28, 2000, 10:58 AM
Obviously, my above post about known size ring should have said ".308 (not .038) ring for .308 bbl" Fingers aren't doing what my brain says lately.

Yup, I've used air gaging quite a lot in the past but never for bbls. BTW, air gaging can be used for flatness & other type measuring applications as well. It's pretty versatile & the equipment can be pretty expensive.