From: sfaber@intgp1.att.com
Subject: AR-15 Building hints
Organization: AT&T

Here are some hints on things I ran into building AR-15s that are not
covered in the FAQ.

Jamming problems:
There are certain things to look out for here.  Many people prefer
to buy only Colt bolts, or GI issue parts and are afraid of other
aftermarket parts such as offered by Nesard and others.
I believe that if you carfully inspect some critical parts you
can build a parts gun that is as reliable as any.
Jamming can occur from short cycling, poor extraction or ejection.

Binding bolt carrier:
First check for binding in the bolt carrier.  With the charging
handle in place and the upper detached, slide the bolt carrier in
and out without chambering any rounds.  The carrier should slide
in and out very freely without requiring much force.  Bad carrier
keys are common and have been known to cause binding that will
cause a lot of wear on the upper and lead to jams.
Make sure the buffer, spring and tube are properly lubed.  The
buffer tube should be smooth with no lathe turning ridges.
With the weapon assembled, pull the charging handle back and
note some force is required to cock the hammer.  Then let the
handle back and repeat.  There will be some dragging of the carrier
on the hammer, but it should not require much force.  Try again with
keeping the trigger pulled.  This time the disconnector will keep
the hammer pulled back even farther, so the dragging will be very
light but still there.  There may be a problem with the lower/hammer
if too much drag exists.

Ejector tension:
Bad ejector tension is a major stumbling block that can be very
frustrating and has lead people to sell their guns or by Colt bolts,
or pay gunsmiths a lot of money.  Aftermarket bolts are frequently
assembled using the same springs for the ejector as is used as a
detent spring elsewhere on the gun.  The problem is that the
ejectors themselves vary in length and possibly the holes in the
bolts do too, so when these springs are used, they usually create
excessive force on the ejector.  The gun may appear to function
properly but after a rapid fire string in the summer, cases will
start jamming up into the gas tube groove in the upper receiver.
At a DCM match this then leads to a number of helpful people poking
at your gun with screwdrivers trying to get the case out.
What has been observed is that a strong ejector tension will kick
the brass out at about 5 o'clock 10-12 feet back.  The gun will
mostly function OK until the spring heats up and loses tension.
The brass will not kick out as far and then jams occur.
Proper ejector tension can be achieved by cutting or grinding the
spring so there is practically no tension when the ejector pin is
installed.  The brass will then typically kick out about 2-3 ft at
2-3 o'clock, and there will be no more jam problem.
Apparently when the ejector tension is very high the gun will
operate, or low it will operate properly, but in between it will jam.


Short cycling:
If rounds are sometimes not chambered it is due to the bolt not
coming back all the way.  This also causes jams since the case may
not be ejected.  The problem is either something is binding
or the gas system is leaking or plugged.  The gas tubes are
supposed to be self cleaning and seldom plug up unless perhaps you
are using wimpy loads that produce a lot of carbon.
It is important to know your gas system and where the possible
leakage points are:  near the front sight, at the carrier key,
at the gas rings, and where the bolt extends through the carrier
by the firing pin.   There will typically be some leakage at the
front sight as seen by some black deposits, but if the tube fits
tight it is probably OK.  The bolt post to carrier hole clearance
is typically very tight - less than .5 mil and I have not had a
problem there.  If the bolt does not fall out of the carrier when
you invert it, the gas rings are probably OK.  Gas rings will rotate
when you fire the gun and are not like car rings that seal into
position.  Sometimes people experience problems when all the ring
gaps line up, but I have not had that problem.  It is a good idea to
separate the gaps each time you install the bolt anyway.  The biggest
source of gas leak trouble I have found is the carrier key to gas tube
contact.
Carrier key:
There are a lot of bad carrier keys out there.
This is the one item that should be looked at very carefully.  Not
only are they a source of potential bolt/carrier binding, but they
can cause excessive gas leakage if the fit to the gas tube is bad.
Gas tubes have a little ridge on them where they fit into the
carrier key that determines the degree of gas leak sealing.
All the new gas tubes I have seen have a quite uniform diameter
ridge and have not been a problem except when they get worn down.
The ridge can wear down a couple mils which then can cause
excessive leakage.  I have had that happen after 1000 rounds or so,
but the quality of your carrier key can definitely affect this.
My first carrier key caused binding as described above.  The next
one had a bit of a ridge inside increasing the inside diameter
after a point.  This caused the worn gas tube and excessive gas
leak problem.  The next key I selected to make sure it was uniform,
symmetric, and had a nice smooth inside surface that made a good fit
to the gas tube, and this one has lasted a long time and is still
giving good service.   Even supposedly GI issue carrier keys I have
seen can have fairly rough inside surfaces, so pick them carefully.

Headspace:
The FAQ makes a lot of hupla on headspace.  There are a few things
that should be pointed out on this.  If you look in the Forster
catalog for example, you will see the following for .223 Rem:

Go	No Go	Field	Abs Max
1.464	1.467	1.470	1.474

If we call 1.464  0 headspace, then the others are at
+3, +6, and +10 respectively.
Originally M-16 armorers had no-go and field gages, but recently
they went to one gage which I am almost certain is the Abs Max
+10 gage.   So I would not be overly concerned if your rifle closes
on a Forster No-Go gage.   Typically the headspace on a rifle will
be around +3 to +5-6 or so and they will be fine.
Another point is that the armorer gages are machined down on the
edges of the base so no contact is made with the ejector so no
tension will be applyed due to this when the bolt is closed.
The proper way to operate the gage is to load it in the chamber and
let the bolt/carrier drop slowly into position under its own weight
and not to push it down under pressure.  If the bolt does not close
all the way, then you know the headspace is below that of the gage.
You can easily be fooled by several mils if you force the bolt
closed.  This is more of a problem with normal Forster type
headspace gages that have flat bottoms that would put the ejector
under tension.  Also if you use a Go gage (minimum) and add paper
or other spacers to simulate No-go gages you have to be careful not
to put too much force on closing the bolt or you will be fooled
like I was once about your headspace.
Another way to measure headspace is to get a RCBS Precision case
mike.  After you get it you probably will have to send it back to
get it adjusted properly for 0 at 1.464 in. from recent
experiences, but then you can resize a series of cases and measure
the headspace of them and select a set that range from 0 to +10 mils
and you then have a set of headspace gages.

Steve