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Can you show/explain how you got or get the barrell extension on the barrel and how you got the head-space to be just so? I have always been confused about that process.

Great pictures!

Mr. White,

It's all about math. You measure the extension depth minus the lug thickness from the back of the extension. I'll write a better explanation here in a few.

Adam

http://firearmsdesigner.com/?p=153

Mr. White this is where I found most of my info.

Adam

Ok, I have a question for you AR gurus. Are MilSpec bolts all similar? I mean to say this: could I use a seperate bolt to headspace and chamber? I know that people slap on uppers and bolts eveyday with no issues.

Adam

Thanks for that link! I always wondered how that was done. Great read.

I hope an expert will give you an authoritative answer on this one. The only way to adjust head space is to remove the extension and adjust the cut on the shoulder where the barrel extension hits... correct? Or is there something that can be done to the bolt? I have always wondered. With all the barrels being sold for do it yourself assembly I had always assumed that the bolts were all made to tight consistent tolerances as well as the barrel extension installations at the barrel manufacturers. From what little I have read here and there folks can double check head space with go and no go but I never read anyone say how they corrected it.

99% of us can't correct the headspace if it fails the gauges. It's time for a new barrel or bolt, or both.

Correct. Insert the bolt into the extension and measure. This gives you your actual depth. I add .005" to that to compensate for crush when torquing the extension on.



I would say factory tolerances are loose enough to work withing the specs of military tolerances. Much like SAMMI specs.

Adam

Once headspace is set, barrel extension pinned, the gas port is drilled TDC and you better hope your bolt matches the one it was set to. The only way to change it is to remove the barrel extension, cut off the exact amount of barrel so the gas port lines up with the pin at TDC and rechamber.

I've never seen a new bolt not pass the field gauge but I still check when buying a new barrel.

If your headspace comes up too short, run a reamer in and turn it by hand. That will give you the few extra thou you need, because you ain't going to get anything swapping bolts.

You can grab random bolts from 5 different vendors an not find a thou and a half difference between them. From the same maker? Chances are they will all be within a half thou. Most places chamber close enough that they grab bolts out of a bin and don't even bother with a gauge.

If you come up too long, well, that's a problem as others have pointed out.

Got a little more done today.



Adjusting the shank length



Shank adjusted and headspaced



Final shank length, time to check chamber depth and ream accordingly.



Go gauge to check headspace.



The nice thing, when I face off the barrel it almost was perfect protrusion. In this case .131" with the Go gauge.



Once the chamber was cut to proper depth, I put a 29* cone @ .570" wide for proper feeding.


Tomorrow I'll torque the extension and drill the gas port. I may or may not put a compensator on it.

Adam

I also have to turn down the shank to 1". Adam

I set of the muzzle and indicated it in. from there I did a small floorboard crown and polished to mirror to ensure no burrs or scracthes.







Next I set the barrel between centers and prep'd for cutting the shank to 1".







Good results.



Tomorrow I have to turn the barrel down as the gas block will not hold steady enough to taper the hole, win some lose some.



Adam

Well today was a slow day, I got to work on the gas block. I reamed it from .625" to .675" this way I don't have to contour the whole barrel just a seat at the gas port.



Adam

Well finished it up today, well as much as machining goes. I had to turn the gas seat (PITA) and then I torqued the extension on. I set it up in V Blocks and leveled it, used a pin to find top dead center and then moved down until I hit the mark I transfered to drill the port. I used a center drill to peck the starting hole. Then I started with the .060" drill bit and moved up to .073", bur free and right centered on the land and groove. After that I used the barrel spinner and cleaned up the barrel.

Now I just have to paint and test fire.

Adam

Adam looking good! Thanks for the pictorial.

Really enjoyed this thread.

Looks like an extremely short gas system for that barrel length. Any reason you chose that length?

It's not too bad on length, but it's shorter than I expected. I ordered a rifle length gas system and when I received it the tube measured out to 12". It's was the correct length but shorter than I anticipated. I really needed the 14" tube, so I compensated with a smaller port hole. All of my research yielded no informant on barrels over 20". Since this is a learning experience and not a production rifle I figure of it gives me fits I'll chop a few inches off the barrel. On that same note I couldn't find any contradicting information that said I couldn't go with mid length tube and a 23" barrel. I found information on AR15 barrels dealing with pressures in the 223/5.56 and with this system I figured I'll be running about 20k psi at the port since I used a port .010" smaller than recommended. Again if my dwell time is too long I'll chop and re-crown.

Adam

20k? A rifle gas on a 20" barrel is smooth because it runs at 13.5k.

Interesting.