I was looking though some of the dealers web sites and noticed that one was selling M4 barrel extensions. I was wondering if there were any good reads that anyone knew of that explained changing barrel extensions and correcting headspace?

It is very hard to swap extensions from one barrel to another.  There is no standard as to where the threads start on both the barrel and extension.  If they don't start just like your old extension, your barrel index pin will not be aligned with your gas port.  If you had acces to a large number of extensions, you probably would find one that would torque lined up properly aligned, with a little tweaking.  Headspace must be checked and adjusted properly during this process also.

In case you don't know, there is a special extension sockey used to install and remove barrel extensions.  This costs around $65.

The easiest route, and obviously the most expensive route is to buy a barrel and receiver configured for the M4 extensions and build from there.

Good luck.

I'm currently going to machine tool classes and have access to lathes, mills and alot of mesureing intruments and thought that if I knew how I might buy a cheap used barrel and give it a try.

Quoted: I'm currently going to machine tool classes and have access to lathes, mills and alot of mesureing intruments and thought that if I knew how I might buy a cheap used barrel and give it a try.

You greatly increase your chances by having access like this.  Most don't and think they can just swap one for the other and go.  Just remember at the end, proper headspace is what your MUST end up with.  Improper headspace can grenade a rifle and you'd be lucky to escape injury should the worst happen.

Good luck.

You need to look at some drawings and understand how 'headspace' is accomplished in these rifles. It is a literal bitch to try to retrofit a barrel extension to a pre chambered barrel.  It is much easier to install the extension, drill the gas port (lining up both the extension and port, so the port falls in a groove, not half way between or on a land) and then ream the chamber to the proper depth. Indexing of the extension is accomplished by setting back (cutting away) the shoulder on the barrel that it butts against.   Headspace is set by gradually reaming the chamber to a depth that will allow the bolt to lock into the extension with a 'minimum' gauge in place. These dimensions can be set by accurate measurements, but the foregoing description is by far the easiest for the novice builder.

I don't think it was ever intended that a barrel could be retrofitted with a new extension, but only that an extension could be put on a new barrel blank--THEN the machinist would locate and rill the gas port, index pin, etc.

I've tried it, good luck, does'nt work unless you don't mind sloppy headspace and bending a gas tube like spaghetti to get it to line up right.  As stated the factory does it in this order, Thread barrel for extension, Chamber barrel blank, install extension, drill index pin, drill gas port.  A CNC machine can rough and finish ream a barrel so that when you install the extension the headspace is perfect or very close.  As stated the barrel extensions and threads on the barrel are not indexed the same every time, because the barrel threads are done before anything else.  Just buy a new barrel, you'll be glad you did.

Tool, $65, extension, $20, chamber reamer (what does that cost?), and you still have not figured in aggravation, other machine work.

You can BUY a M4 type barrel for under $200.

And what is the matter with your extension?