From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Location of the gas port.
Organization: Hewlett-Packard Fort Collins Site

Accuracy tests with M1 rifles in the 1960s proved the gas port should
be in the groove if accuracy is the first objective.  When the port is
on a land, the flash caused by drilling the hole tends to scrape off
bullet jacket material thereby further unbalancing the bullet.

Bullets ride the lands, or bore diameter.  Top-quality barrel makers
have known this for many years.  The lands need to be very uniform in
both shape, bore diameter (top of one land across the bore to the top
of the opposite land, or that same plane in 3 or 5 groove barrels) for
minimum bullet deformation or unbalance.  Groove diameter is a tad less
important, but still needs to be very uniform.  When a tiny flaw, such
as the near microscopic flash from drilling a gas port, is in the groove,
the amount of bullet jacket tear is virtually zero.  The bullet is not
unbalanced enough to be detected.

If the gas port is drilled in such a way in the groove that its inside
edge is too rough, then enough bullet jacket material will be scraped
off and the bullet will not shoot accurately.  This tends to be the cause
of inaccuracy in many rebarreled gas-operated service rifles; both M1s
and M14/1As.

Properly set up on a relief-grooved mandrel, the barrel is held in a
drill press or vertical mill aligning the groove to the drill bit.  After
the initial hole is drilled, it is reamed or redrilled to the required
diameter.  With tood tooling, no additional smoothing of the hole in the
land need be done.

BB