From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Newsgroups: rec.guns
Subject: Re: Barrel Dia. measure w/ air?
Date: 12 Apr 1994 00:38:40 -0400

Air gaging of rifled barrels has been happening for about 60 years.
A company by the name of Sheffield in England invented the process.
Here's how it works.

For a given bore diameter and groove count, a precision plug is
put in that has a few ten-thousandths of an inch clearance.  Behind
that plug is a highly-regulated air pump.  That pump pushes air into
the bore alongside the rod holding the precision plug gage.  As the
air goes between each half of the plug, it escapes through the very
thin area between the plug and the bore/groove.  As pressure is 
regulated very precise, any increase/decrease in bore/groove diameter
is going to change the pressure between the plug halves.  This change
in pressure has been calibrated to read on a gage that looks like a
thermometer to parts of thousandths of an inch in relative dimension.
The air gage reads variations about a norm which is the plug diameter.

Many of the top-quality match-grade barrels are so gaged.  As tight
spots are identified by higher pressure pushing the gage to a higher
reading, the are marked on the outside of the barrel next to a pointer
that's even with the gaging plug.  Later, that part of the barrel will
be lapped out to get it to where the maker wants it.  Barrels so made
will have bore/groove diameter varitions of less than .0001-in. and
are very, very accurate.

Some barrel makers, such as Douglas, do air gage their barrels after
they're rifled, but don't lap the tight spots to get the bore/groove
dimensions uniform for the bore's full length.  These barrel makers
just use the air gage as a yardstick; those that are uniform to some
tolerance are graded higher than those that aren't.  Such barrel
makers typically have bore/groove diameters that even when very uniform,
they are too big in diameters for the bullets they use to be accurate.

So, anyone saying a barrel is air-gaged is simply saying the bore/groove
was measured.  Period.  It means nothing as far as accuracy is concerned.
If the bore has been lapped to very uniform dimensions, then the barrel
is probably a good one providing its bore/groove diameters are no more
than .0005-in. smaller that the bullets it's intended to shoot.

Air gaging has one shortcomming; it does not detect out-of-round bores.
Which lends credence to star gages.  Star gages are mechanical, deep-hole
micrometers.  They also measure to the same tolerances as airgages.  But
their claim to fame is that the multi-point gaging parts can and do show
when the barrel is out of round; a key part of good barrels if accuracy
is the objective.  Star gages are nothing more than a precise tapered
rod pushing outward three or more tiny balls against the bore's walls.
The end sticking out of the rifled blank is just like a micrometer but
graduated in 1/10,000th of an inch or less.  

As a rifle barrel can have one of three common methods used to put the
hole in the blank and rifle it, each method typically causes the bore
to vary somewhat in diameters.  Steel has hard and soft spots; the better
the steel, the more homogenous and uniformly dense it is.  Cheap steel
is almost like brick-and-butter regarding its soft and hard spots.  All
of which means that after the barrel blank is rifled, measuring that
hole with a air or star gage only tells one how uniform it is.  Different
barrel makers have different specifications their products must meet.
After spending $60 for a solid, stainless steel round blank, deep-hole
drilling it, reaming and lapping that hole to tight specs, then rifling
it, gaging and lapping and gaging and lapping......until it's perfect
means the barrel maker gets to sell someone a tack driver for over $200.

Air gaging a barrel takes about 5 minutes with a $3000 tool.  Star gaging
a barrel takes about 10 minutes with a tool costing the same.  Lapping
the bore/groove to less than 50 millionths of an inch uniformity from
end to end takes 1 hour or more with $30 worth of tools.  Dozens of USA
companies make rifle barrels; you  can count the number of folks who
can lap them to perfection on two hands.  Such is life. . . . . . . . .
Then you gotta pay someone to shank, chamber, and crown it for your 
rifle action; more bucks someone gets.  A few barrel makers end up with
a better quality rifled blank before it gets lapped to perfection than
other end up with ready to screw into an action.  Accurate barrels are
not cheap; cheap barrels are not accurate.

The marketing of `air-gaged' barrels has most shooters thinking that if
it's air gaged, it must be the best.  In reality, those air-gaged barrels
have, for 99.999% of them, just been measured.  But they don't tell you
how uniform the measurements are and the probably don't tell you what
the bore/groove actual dimensions are, either.  'Tis such a tragedy that
most of the dealers selling air-gaged barrels do not have the slightest
idea of what air gaging really is; star gaged either for that matter.

BB