I have a raw Colt AR15 barrel that I purchased at a gunshow.(Second one I found, I'm getting good at these gunshow deals) It has no barrel extension on it. I have a replacement barrel extension and indexing pin. Where can I find DETAILED instructions on headspacing? If its in one of the manuals, which one? I can complete any other assembly or disassembly operation except this and I want to learn how its done. I also have the necessary guages. Please help! Thanks  

If it's already had a barrel extension installed, and the gas port hole drilled, then about your only option is to torque the new barrel extension on until it lines up with the old index pin hole (TDC, and the TDC for the gas port as well), and check it to see if it headspaces correctly.

If it doesn't, you're SOL unless you want to weld up the old gas port and front sight base grooves, and take a chance on torqueing some more (or backing off) on the barrel extension to get the headspace right, and hoping that there's enough room between the face of the barrel and the back of the extension for the bolt to fit, rotate and lock.  If so, you'd then need to drill a new gas port, index pin hole, and then drill for the front sight base taper pins again with the new TDC.

If the chamber's been chrome-lined (as almost all Colt barrels are), then you'll screw it up trying to shave the face of the barrel and re-ream to correct depth.

Basically, unless the new barrel extension goes on and headspaces correctly at the old TDC, it's probably more trouble than it's worth.

Here ya go: [url]http://www.jobrelatedstuff.com/forums/topic.html?b=3&f=7&t=142139[/url]

It will be pure luck if you install a new extension on this barrel to the correct torque (150 lb-ft) and get any predrilled gas port or sightpost taperpin holes to align correctly.

Save yourself the agony and expense and buy a complete barrel assy ready to install.  The machine shop / gunsmith effort necessary to get this right will exceed the cost of a complete, ready to install assembly.  

As Circuits said, if it's a chrome plated chamber you're screwed.  They can't be readily finish reamed if you end up with a short chamber (too little headspace).  If the chamber is long (too much headspace) you can machine the front of the extension or back of the barrel shoulder to get the right headspace, but you still will have a one-in-a-million chance of the gasport aligning with the index pin.

Buying predrilled "naked" barrels cheap is false economy.