I really think I would like to try making a AR15 barrel from a 1 1/4" blank.  I have access to the proper machinery and tools but I lack the experience.  In particular I need help in figuring out how to attack the barrel extension.  I know I need to thread the barrel for the extension and that should be no problem but what about the torque spec?  And what are the specifications for the index pin?  

I can chamber and headspace a mauser barrel but the headspace on an AR will need to be checked after the extension is in place?  So how do I know how deep to ream?  Keep putting the extension on and off?

Anyway I am starting to reseach on the web but if anyone has the answers to this or knows of a good book or website please chime in.

Thanks!

The shank is .810x16, 60 degree V thread and it's about .620 long.  You can calculate the shank length needed using your bolt and extension.

When the chamber is close to depth, put the gauge in and screw the extension on with the stripped bolt in place.  When everything is snug, use a feeler gauge to measure the gap between the extension and the barrel shoulder.

Thats how much further ya got to go (less a few thou for compression/clearance).  Just like doing a Mauser while looking in a mirror.

Drill into the threads for the index pin and tap it in.  Drill deep enough to leave the top level with thr receiver ring.  Drill the gas port at the same time without changing your setup.

I forget the torque.  225 seems like a nice round number.  Takes a quality barrel vise and stout extension wrench.  DPMS used to sell decent extensions and wrenches.  

175 Lbs on the extension to barrel.

Read this as a primer,

[url]http://www.compasslake.com/barrels.htm[/url]

Someone posted a slick way before.

Basically, cut the chamber using your bolt as a guide.  Stop when there is minimal clearance between the bolt and the barrel end - I recall only a couple thou clearance, if that.

Cut the external threads, and cut the clearance for the shoulder well short of where you expect the shoulder to be.

Do your final extension fitting with a headspace gage, your bolt, and the extension, taking just a bit of that extension shoulder until you end up with a perfect fit.

Tighten as you see fit, and drill and pin the extension into place.  

EDIT:  That Compass Lake website is nice, but it doesn't really tell you how to do it yourself.  That page on fitting blanks is full of fluff.  Not much useful info.

That looks like a good system.  You will shorten the headspace dimension about .002 when the torque is applied to tighten the extension.

Allow for that when the chamber is cut.  If you want .001, cut to .003.

There is a NRA re-print M16/AR15 publication floating around that details the USMC Quantico Armorers building barrels for match use. It includes a decently done drawing showing the thread details. The text is very well written also.

The threading for chamber end is .8125"-16 x .620".  Torque is 150 ft-lb.

Guys, Thanks for the help.  I have already cut threads on scrap stock for practice waiting patiently for my barrel extension to come in.  

I'm pretty excited about this.  I am going to post in the pistol caliber forum about my next question.  The blank I intend to use is a 45 and I am going to try to build my own 45 ACP upper.  If you have any ideas check that post out for me.

Thanks!

Just curious, why would you want to use an 1 1/4" barrel for a pistol round chambering??

I intend to profile it, 1.5" is the diameter of the blank I have to work with.  It is a free gift so I'm trying to figure out what to do with it.

If you are going to make a 45acp barrel in "Colt" pattern, you do not need a barrel extension, simply cut the breech end to look like to outside dimensions of a barrel extension.

This barrel is for 9mm Colt Pattern.
[img]http://www.imagestation.com/picture/sraid109/p004fda02d4185b9fbd2cc8b0fe5950a3/f934395a.jpg[/img]

Cool, makes sense.  But the more I think about it I don't think I want to tackle a 45 acp because of the many many issues with that caliber in the AR.  

I guess I am dying to make a barrel but only have pistol caliber blanks available to me.  So I might be out of luck.  The blowback nature of the pistol calibers scares me.  

The only thing I can think of that I might be able to do would be a .44 auto mag.  Trim .308 cases and chamber a 44 blank would be easy and could operate as a gas gun but I don't think the bolt is large enough.

Tagged for reference.  May have about 30 blanks coming my way soon.