This is what I have done so far and some questions to start with.
I put the barrel blocks on the barrel, put it in the vise and went to turn it off and the barrel just kept moving in the blocks. Then I took a lead brick I have and cut it in half, and made it into a barrel vise setup, the barrel turned in it as well. So, I did what I would do on a FAL, I broke out the dremel and cut down the length of the bad extention until I could just barely see threads. Once there I took a chisel and wacked it with a 2 lbs sledge, and it cracked along the cut. Before I did this I made a mark on the barrel shoulder that indexed the key to TDC. Then I measured the depth of the extention from the outside to the barrel face, it was .5041. That should be my headspace depth.

Now once it was cracked it unscrewed quite easly there was some rust on the original threads. I hand tightened on the new extention the hole at the top did not come to the original TDC mark I made. So, now the question I have is, where do I reduce to get the original head space and how much in order to crank on at 175 Ft/lbs to get it right?

I could reduce some off the front of the barrel extention, but then I would be changing demision on the shoulder of the extention. I could take it off the barrel shoulder, but that would require a lathe, to get it perfect. Does it matter if I change the thinckness of the barrel extention shoulder, and if so what does it change?
If I turn the barrel shoulder it shouldn't matter other then headspace.

Lastly, what can I do to keep the barrel from rotating as I'm trying to wrench on the new extentioin?

You are in deeper than you think.  You simply cannot change a barrel extension, especially without a lathe, it is not a swap out item.  As you have noticed the thread starts are different and your extension didn't land at the same spot as the original.  You cannot take anything off the barrel extension or the barrel shoulder.  That will make your headspace too short.  The only way to adjust headspace after the extension is installed is with a reamer...................on a lathe.  If the barrel is chrome lined, you are done right now, as you cannot ream a chrome lined chamber, unless you don't want a chrome lined chamber and you want to risk destroying an expensive reamer.

Good luck.

[homer]D'OH![/homer]

You done messed up.

Quoted: Then I measured the depth of the extention from the outside to the barrel face, it was .5041. That should be my headspace depth.

Your headspace measument sounds incorrect.  Headspace is relative to the chamber depth.  Or I may have misinterpreted the above statement all together.

Your best bet is to buy another barrel, chambered, and headspaced with a barrel extention installed.  An expensive mistake, but still a mistake to learn on.  Good luck.

No. Headspace is measured from the face of the bolt when it is in the battery position. You cannot remove material from the barrel extension shoulder because that will change the timing and locking of the lugs on the bolt and the barrel extension. You have screwed up BIG time and you desperately need a gunsmith with a lathe to fix this, if it can be salvaged. Do not 'improve' or 'repair' it any further. Get some assistance,

First off, now I haven't screwed up big time.

I would like some to tell me the distance of the inside of the locking lugs on the extension to the barrel face that would allow for the fireing of surplus ammo.  This is the distance that lets the bolt cam into place and lock up when firing.  Does anyone know this deminsion, becuase it must be the same accross the board whenthe rifle was originally designed.

That number I gave .504 is a depth from the outside edge of the extention to the barrel face, if you subtract the width of the locking lugs on the inside of the extention then you should have a mil. spec. designed distance there.  A gap I should say that would allow for the fire of different manufacturers of 5.56 ammunition.  This isn't some mystic rifle I design stuff all the time someone designed this with a fixed nominal distance.  All I have to do get to that distance and every mil. spec. bolt I put in it should fit.  Enough defeatism I just need the numbers.

Lastly, I do not see what what it would hurt to take .0025 off the extention face to get it to time correctly.  Tell me what is so wrong with this thinking?

Quoted: Lastly, I do not see what what it would hurt to take .0025 off the extention face to get it to time correctly.  Tell me what is so wrong with this thinking?

If that leaves you with tight headspace, how do you expect to fix that?  You're extremly lucky you're even that close.

Here's your headspace measurements for GO, NOGO and FIELD for .223 (close enough to 5.56 for discussion, if applicable):

GO = 1.464"
NOGO = 1.467"
FIELD = 1.470"

From here.

You have, for the sake of argument, 0.006" to play with.  If you were tight initially with your old extension, as in a new barrel, and you take 0.0025" off of your new extension, you could be tight, dangerously tight if you try to shoot with it.  Just an FYI.

Additionally, if you have any variance in the barrel extensions, your measurement of the extension from the barrel face means nothing.  You are assuming that the extensions are identical, both internally and externally.  Not too bad of an assumption, but when you are counting in thousandths, it could add up.

Good luck.

Got to ask. Why did you remove the extention?

Quoted: Got to ask. Why did you remove the extention?

I'd guess he wanted an M4 extension.

No reason to spell it correctly. Read the title

www.dpmsinc.com/10Expand.asp?ProductCode=TL-BX
Here a link to the tool to tighten it.

RYNICH you and your assumptions about barrel headspace and  bolt timing are beyond belief and defy explanantion. Charles the Gunsmith.

"if you have any variance in the barrel extensions, your measurement of the extension from the barrel face means nothing."
Yes it does mean something, if the back of the lug which cams against the back of the lug on the bolt, are of the same deminsion, then you could check the headspace from the back of the extention to the barrel face.  THis is the distance I speak of the lugs on both extentions new and old measure the same, from inside to back of extention.

The inside of the locking surface of the cam area, to the barrel face does.  This is your headspace gap on an M-16/AR-15.  If I had pics you would see what I'm talking about, the bolt cams in a recess, which locks it inplace upon the back of the shell casing AR-15 101.  There was designed a maximum and a minimun for this recessed space from the inception of the design.  This recessed space is what sets your headspace.  I know how these barrels are put together from the factory.  I'm trying to do something that yes, is out of the norm but not impossible.

I removed the extention because some numbnuts before me distroyed the feed ramps and removed them, making the barrel worthless as it was.  I liked the idea of challenging myself on something that as far as I know no one tries.  Using my gunsmith knowledge I looked at the way it is put together and determined it is quite possible to install a new extention.  

I have nothing to loose and very little money in it, remeber this is a challenge of my own abilities, and the information I can get from you will help.  I'm obviously not going to fire something I know to be unsafe.  I was trianed as an armorer in the US ARMY.  I do know a little about this design, so helpful information is what I want.  From people with experience trying this.

I already know how crazy I am, so we can stop with comments to that effect.

Lastly, if it doesn't work this is going to become a 22LR conversion barrel/upper.  So, as you can see there is nothing to be lost by trying.

Quoted: "if you have any variance in the barrel extensions, your measurement of the extension from the barrel face means nothing." Yes it does mean something, if the back of the lug which cams against the back of the lug on the bolt, are of the same deminsion, then you could check the headspace from the back of the extention to the barrel face.  THis is the distance I speak of the lugs on both extentions new and old measure the same, from inside to back of extention. The inside of the locking surface of the cam area, to the barrel face does.  This is your headspace gap on an M-16/AR-15.  If I had pics you would see what I'm talking about, the bolt cams in a recess, which locks it inplace upon the back of the shell casing AR-15 101.  There was designed a maximum and a minimun for this recessed space from the inception of the design.  This recessed space is what sets your headspace.  I know how these barrels are put together from the factory.  I'm trying to do something that yes, is out of the norm but not impossible. I removed the extention because some numbnuts before me distroyed the feed ramps and removed them, making the barrel worthless as it was.  I liked the idea of challenging myself on something that as far as I know no one tries.  Using my gunsmith knowledge I looked at the way it is put together and determined it is quite possible to install a new extention.   I have nothing to loose and very little money in it, remeber this is a challenge of my own abilities, and the information I can get from you will help.  I'm obviously not going to fire something I know to be unsafe.  I was trianed as an armorer in the US ARMY.  I do know a little about this design, so helpful information is what I want.  From people with experience trying this. I already know how crazy I am, so we can stop with comments to that effect. Lastly, if it doesn't work this is going to become a 22LR conversion barrel/upper.  So, as you can see there is nothing to be lost by trying.

It's your barrel.  I'm just trying to show you some of the pitfalls.  You're a trained armorer, but IMO, you are heading for potential disaster.  You are making alot of assumptions in the reference measurements you are using.  I've done this before and I know what works and what doesn't.  Just sharing my pain, that's all.  Very few people can do what you are trying, so asking for "how to" here is almost pointless.  You are essentially on your own, in "no-man's-land".  TIFWIW.

Unless I'm mistaken, headspace isn't just the barrel extension spacing.  It's also has to do with distance to the chamber shoulder.  My understanding is that it is, essentially, the distance from the bolt face to the chamber shoulder.  Why else would you measure it with a headspace gauge that goes to the chamber shoulder????  I would think a trained armorer would know this.

And I quote:

In simple terms, headspace is the dimension of the chamber of your rifle, the gap between the face of the bolt and the stopping surface for the cartridge. To be precise, it is the distance between the face of the bolt and the datum line, which is a circle of stated diameter, along the slope of the shoulder of the cartridge.

From here.

Here's what you need to do, IMO, and this is by no means the "correct" way, but one that makes sense to me:  

Take whatever you think you need to off the barrel extension and mark that down.  Remember that when you torque the extension, it will move - alot more than you think, so you had better do some trials with it now, to get a feel for how far it moves and factor that into your calculations.  

Once the extension is installed and torqued, check headspace.  I'm not sure exactly how to check for short headspace, as there is no formal gauge for that - the GO gauge only tells you that you aren't too long, IIRC.  I'd chamber a few live rounds - carefully - or get some dummy rounds that are dimensionally correct.  If they chamber, you might be OK, if they don't, you know you are short and need to ream the chamber longer.  For this you need a lathe.

When/if you finally get to firing live rounds, I'd pay particular attention to signs of overpressure.  If you are short, you will see it.

Good luck.

Thanks this is the kind of info I was looking for and I appreciate you sticking with me on this.  I think you see what I'm going for.  Like I said if it doesn't work it will become a 22LR conversion uppper and that doesn't really matter if it is TDC.
I understand the normal way of headspacing and the fact that it includes chamber length I just didn't mention it because I assumed it would be a given.
You are correct in that aspect the other part of headspace on an AR-15 is the bolt lock up, worn or broken lugs can cause excess headspace leading to out of battery operation enough so to cause a bad day, in other words unsupported shell banding.  
I hope to get the spacing correct compared to the original, I also do not have bolt/bolt carrier yet to check with the headspace with.  
I had planed on doing some test runs to see the distance of turn at 150ft/lbs before I reduce the extention front (portion that fits up against the barrel shoulder).

I'm starting to think you are correct that Iam in no mands land with this project, that is why I was trying to share my ideas and theories of how it could be done, not whether it was possible or not.

I guess that there are some that think there guns are some precious jewel, I see the black rifles as metal, and done safely can be worked over.  I like to keep my head and face the way it is if I think it is unsafe I will not try it, with out proper safety setups.

I'm no fool just hacking on guns, I have studied the design and what I'm doing is not impossible it is just hard and with a llittle luck I think I can make it right.

If not I will post what I come accross I'm trying to photo document what I have been doing.

It's all part of the learning curve.

What are you going to tighten the extension with? Since you have one apart what size are the threads on the barrel? Is the chamber chromed or not?  How far off is the extension from the correct postion? Just curious no comments. Thanks.

Rynich, I sincerely hope that you get it figured out. If this was my barrel and I wanted to truly salvage it at all costs, I'd torque the barrel extension on at whatever position it ends up. Then I'd remove the FSB, measure the gas port in the barrel, then weld up the original gas port. Then I'd redrill a new gas port in the correct position, and redrill the two holes for the FSB taper pins. Difficult? Yes, but nowhere near as difficult as trying to reindex the barrel extension.

"What are you going to tighten the extension with? "
I'm working on that.

"Since you have one apart what size are the threads on the barrel?"
I will find out, I don't have a thread pitch guage.

"Is the chamber chromed or not?"
Chromed, so no I can't ream it.

"How far off is the extension from the correct postion?"
I 'm not sure what the factory correct position is, I'm trying to find out.  the original from outside the extention to barral face was .5041 the new one cranked on and the TDC hole at 3 O Clock I get .5065.  A difference of .0024.

M4,
I appreciate your suggestion, I thought of that as well but I was concerned about blowing the TIG through to the bore.  If what I'm attempting fails I will just make it a deadicated 22LR barrel, the conversion doesn't use the feed ramps on it anyway.  I'm in no hurry to get this done, I'm just trying to collect info before I do  anymore work.

Tag

Quoted: "What are you going to tighten the extension with? " I'm working on that. "Since you have one apart what size are the threads on the barrel?" I will find out, I don't have a thread pitch guage. "Is the chamber chromed or not?" Chromed, so no I can't ream it. "How far off is the extension from the correct postion?" I 'm not sure what the factory correct position is, I'm trying to find out.  the original from outside the extention to barral face was .5041 the new one cranked on and the TDC hole at 3 O Clock I get .5065.  A difference of .0024. M4, I appreciate your suggestion, I thought of that as well but I was concerned about blowing the TIG through to the bore.  If what I'm attempting fails I will just make it a deadicated 22LR barrel, the conversion doesn't use the feed ramps on it anyway.  I'm in no hurry to get this done, I'm just trying to collect info before I do  anymore work.

The easiest way to tell how far off you are with the extension is to compare the index pin to the gas hole.  When tight and torqued, these have to be perfectly in line.  You could measure all day long, if the gas port and index pin aren't in line, you rifle will not function.  It has very little to do with your measurement.

How much of one rotation takes you in and out of the min and max headspace range?  Is it something like only 55 Degrees?

Mongo,
Before I removed the extention I marked the original position of the index pin, this is why I said the new one is at about 3 Oclock in reference to my index mark.  I'm attempting to get the extention index pin tightened around to the point where the original index mark is located.  At the same time keep the cam distance inside the extention from becoming to short, thus causeing too tight of a head space.  If all works out I hope to index like the original and get the same distance in the cam area, to the get the headspace like the original.

I want to ask, has anyone ever reduced the face and shell pocket on the bolt in order to slightly open up your headspace? Anyone.

"How much of one rotation takes you in and out of the min and max headspace range? Is it something like only 55 Degrees?"

I'm going to try to get the thread pitch today then I can calculate the forward distance in one revolution.

I didn't have time to get to it last night and I probably won't tonight.

I want to ask, has anyone ever reduced the face and shell pocket on the bolt in order to slightly open up your headspace? Anyone.

Would you not loose the heat treatment. I think the bolts are 8620 ? Also firing pin protrusion?

Are the threads 13/16- 16UNF ?

Barrel extension (BE) threads are 13/16 x 16 TPI. This measures 0.8130 inches diameter on a micrometer. At 16 TPI that means that each thread is a length of 0.0625 inches. Divide 0.0024/0.0625 times 360 degrees and you will know how many degrees it will move. I would have thought an Army armorer would know this. I am a retired Marine armorer and we were taught this at the armorer schools.

Its been a long time, and if you don't use it you loose it atleast that how it works for me.

I forgot about the fireing pin protusion, your correct that would have to be concidered.

Last I heard, barrel extensions are put on, head-spaced, then the gas port is drilled, indexed to the pin...... Make a .22 out of it.....
my .02

you will know how many degrees it will move.

Do not understand what "it" is moving to? What measurement of distance does this give you?



I was talking how little it actual takes to miss your window of correct headspace.
Would it not be .0625/360=x, x equals the distance of one degree. Then the difference of a go gauge and field gauge divided by x to see the amount of degrees to reach that distance?

Sharpshooter,
I think that is probably the way it will turn out, but in the mean time I'm going to try to get it lined out.
For those that care to know let me not mislead you, I was trained as a unit Armorer as part of the run up to GWI.  
I was not trianed in it as an MOS, we didn't have an armorer so the company commander chose 2 of us to get trained.  Prior to that I was doing fixes to our weapons that I was not officially qualified for.  I think any of you that have served or served in broke dick units understand what I'm talking about.  
I happened to have had my 11BP mos changed to 11MP to suit the needs of the Army, I just happen to be in transition and was made a Mech trooper.  I came to a totally broke unit, so broke they ask me why I had 6 mag pouches and 2 canteens, a strobe, and a compass when I went to load up for field training.  Keep in mind we were weeks from getting deployed to the Kuwait.

I just wanted to clarify my Army armorer background, before some of you assumed to much.  Some my not understand this but then a line soldiers was not qualified to take the FCG out of the M-16A2, only a trained Armorer was Qed for that. Just fyi.

What you are doing is probably beyond any armorer anyway.  If the .mil has a bad barrel, regardless of the reason, it gets scrapped and a new one installed.  There's no reason for any military armorer to waste time on a project like this.

Rynich -
If you really want to pursue this treat the job as a barrel setback of 0ne thread length 0.0625. With a LATHE move the shoulder forward 0.0625, then move the threaded portion of the BE threads forward 0.0625. Then, using a CARBIDE reamer, which is available from JGS, move the shoulder of the chamber forward as necessary to get correct headspace. Holding these dimensions, EXACTLY, will allow the gas port to come out  at the correct 12 o'clock position. You will also have to do some minor work on the handguards to get them to fit as they will now be 1/16 of an inch (0.0625) too long. Good luck. Charles the Gunsmith.

That is what I was thinking of and I figured I would have to shave some off the handguards.  This is a Chromed barrel I didn't think I could ream it if it was?

Carbide reamers are expensive, but, they do cut chrome lined chambers. Charles.

Quoted: Carbide reamers are expensive, but, they do cut chrome lined chambers. Charles.

Without having any knowledge on the subject, but if you ream a chrome lined chamber, with a chrome lined bore, making a transition from nonchrome lined to chrome lined, how long will the chrome lining last in the bore, specifically in the throat area?  I'd think you would see some type of degradation there, faster than you normally would, but that's just a guess.

Quoted: That is what I was thinking of and I figured I would have to shave some off the handguards. This is a Chromed barrel I didn't think I could ream it if it was?

Like I thought, you're heading down a road of endless mods and nonstandard, out of spec parts.

Mongo -
I would predict that there would be accelerated chrome lining erosion and probably accuracy problems as well from this type of barrel setback. I have done a couple of these and accuracy was generally not as good as prior to the setback. One was experimental on my part, just to see if it  was feasible and could be done. Second was for a very adamant client who just had to have it done. I was, and still am trying, to demonstrate to Rynich that while what he wants to do can be done, for many practical and financial reasons, it should not be attempted. The carbide reamers I referred to cost $200.00 each and he can get a good quality new barrel for less than $200.00. Take care. Charles.

Quoted: Mongo - I would predict that there would be accelerated chrome lining erosion and probably accuracy problems as well from this type of barrel setback. I have done a couple of these and accuracy was generally not as good as prior to the setback. One was experimental on my part, just to see if it  was feasible and could be done. Second was for a very adamant client who just had to have it done. I was, and still am trying, to demonstrate to Rynich that while what he wants to do can be done, for many practical and financial reasons, it should not be attempted. The carbide reamers I referred to cost $200.00 each and he can get a good quality new barrel for less than $200.00. Take care. Charles.

I've done this twice, once on a nonchrome lined barrel, which needed a small chamber tweek after the extension was installed and torqued (and the original gas port had to be abandoned and a new one drilled) and once on a chrome lined barrel, but I had the luxury of 30 or so barrel extensions to choose from and one of them indexed within a degree or two.  I then stoned off a small amount off the extension face, probably in the 0.001-0.002" range to get indexed.  I checked and double checked my headspace and it was good.  I have since fired about 200-300 rounds in this barrel without issue - from Wolf 55gr junk ammo to semi-hot handloads.  I also have a barrel blank I'd like to complete someday, just to say that I did, but my lathe needs work before I can even attempt this one.

Mongo -
I also have the luxury of a bucket of barrel extensions. I have made a lot of AR15 barrels for various clients. I make them all from raw blanks, usually Shilen and Krieger. These barrels shoot some awesome groups. It really is not very difficult to build a nice barrel and it certainly is custom gunsmithing. If you need any help holler anytime. Charles.

Tweak,
I don't know who pulled the wild hair out of your ass, but you are just to cool for me.  Trying to get a rise out of me and wanting to go to the pit, what is your problem?
This is the last response you will get from me, unless you want to change your attitude and try to be helpful.

1911builder,
I really appreciate your opinions and trying to help me out with this.  Last night I reduced thee front of the extention the part that meets up against the barrel shoulder.  The index hole now sits at about 9-10 OClock, I don't think it is going to be at the right headspace/cam distance when I get it TDC.  It looks like this is going to be my designated 22LR conversion barrel once I get it around to TDC.  You where correct about the the barrel setback, I just had to try this.  To buy any expensive tools would not be worth it as a one time shot, like you said I can just use the money towards a new barreled upper.  Keep in mind I still don't have a Bolt to check with, so when I do it might work, but right now I don't think so.  

hey i hate to be a spoilsport but when removing the origanal extension and replaceing it you changed from a possible easy fix to changes from 2 demensions to 4or 5 depending on your luck. 1. it will be almost empossible to ream chamber to correcxt headspace (chrome), 2. yov've changed  lug clocking alignment between bolt and extentsion barrel to realign lugs and headspace. 3 you no longer can make a simple sholder adjustment to to barrel and shoulder because your 30% off . It can be done but i'm a machinest and i'd rather buy a new barrel. next time buy a extension wrench  and cast an acra-glas bushing with rosin powder a half inch breaker bar ,a barrel vice.  i've never had do more than grunt to get any extension off. good luck( i save all junk barrel extentsions and i've saved a few barrels). hope this helps and i'm trying not to be .an a__hole about it. any help you need from me send a msg.  

Quickie outline of how I did this once.

NOTE: I did not even waste a thought of trying to get the gasport to align.

I removed the old extension - then turned back the shoulder about .050" I then made a washer from A2 or O1 ( was about 12 yrs ago, mem fading... ) to match barrel Dia. about .065 thick.

Installed the extension then measured head space. Removed the extension and using a surface grinder brought the washer to just above required thickness (+.008"). Rechecked and grind again to final length.

Gas port was 20-30° off - put in a short plug made from tool steel and redrilled the gasport with a carbide stub drill intersecting the existing hole.

Made brass taper pins, to fill the existing cuts in barrel - turned and polished to fit FSB. Redrilled taper pin holes and hand reamed with tapered reamer.

I admit I screwed up - It was late and I figured I was in the home stretch - I reamed from the wrong side!

BTW I used some Brownells Cold Blue on the spacer before that final assy.

And thats my machine shop version of a Duct tape and bailing wire fix.  

Edit: Does this mean I qualify for a WECSOG Diploma ?

Quoted: Quickie outline of how I did this once. NOTE: I did not even waste a thought of trying to get the gasport to align. I removed the old extension - then turned back the shoulder about .050" I then made a washer from A2 or O1 ( was about 12 yrs ago, mem fading... ) to match barrel Dia. about .065 thick. Installed the extension then measured head space. Removed the extension and using a surface grinder brought the washer to just above required thickness (+.008"). Rechecked and grind again to final length. Gas port was 20-30° off - put in a short plug made from tool steel and redrilled the gasport with a carbide stub drill intersecting the existing hole. Made brass taper pins, to fill the existing cuts in barrel - turned and polished to fit FSB. Redrilled taper pin holes and hand reamed with tapered reamer. I admit I screwed up - It was late and I figured I was in the home stretch - I reamed from the wrong side! BTW I used some Brownells Cold Blue on the spacer before that final assy. And thats my machine shop version of a Duct tape and bailing wire fix.   Edit: Does this mean I qualify for a WECSOG Diploma ?

Nope, not even close.  The fact that you used a machine shop can be compared to pro athletes using performance enhancing drugs.  You have been disqualified from any WECSOG consideration.

The proper way of doing this is to

1)  remove the old barrel extension, then deposit the old extension, new extension, and old barrel in the round file.

2)  Next, order another barrel assembly in the configuration you want.

Anything else you do between step 1 and step 2 is wasted time, money, and energy.

First I have a job, this is my hobby, there is no wasted time brcause the idea that "time is money" doesn't count on hobbies. The barrel extention was free,  I have bought no tools fro this job, but havr had several loaned to me by fellow ARFCOMER's.  
It does to matter to me if Iam trying to keep a weapon in its original configuration, like I said I have not been able to get a hold of a 1:12 twist 11.5" CAR barrel, if you know where I can easly get one then yes I would get one.  
Since I'm not in the AR circles all the time I have not been able to get one.  I work mainly on FAL's, and that is is what my tools are geared up for.
When  looked at the AR design I thought I could apply FAL rules to the AR barrel, but I have found ;
1. This procedure is not done very often, and the Gas Block is the timing focus.
2. because it is a rotating block design rather then a shoulder locking design you must take other aspects in concideration in order to get the headspacing right.
So, if I don't get this to come out right I will remove it and take material off the barrel shoulder instead of the extention face.  Buy aa new extention and should be able to get the headspace correct.

Rynich, FWIW
I deal with similar issues regularly when rebarreling with a previously installed barrel or correcting headspace on the AR18 and 180 rifles. Those rifles do not have readily available or replacable barrel extensions though, but they do use non chromed barrels and can be easily reamed for headspace if it ends up tighter than wanted. I have a fair amount of tooling for this work and cannot immagine doing without it, but here goes. (All of the following is beside the point if you could find an extension that would torque in with the gas hole at 12 0 clock.)
What you really need to do is make sure your replacement extension measures the same from the front of the lugs to the surface of the extension that torques up against the shoulder of the barrel. You should use a depth mike for this to assure accuracy of the measurement.
That done, torque your extension onto the barrel. 175 Seems alot of torque since the 18/180 rifles use 125. Your resulting headspace should be as it was with the extension you removed or very close. You really ought to have an extension tool that is spline like and fits into the lug recesses for torquing. I clamp barrels in a hole drilled into a split aluninum block that is put in my press and around 5 to 7 tons put on it. Some powdered rosin can be used if it tends to slip, but that is unlikely with this rig.
Assuming the gas hole does not end up at 12 o clock, hopefully it may be at 3 or 9, or somewhere close as that will make redrilling/ repinning easier.  Redrill a new gas hole as needed and remount your front sight. You can ignore the old gas hole as it will fill up with carbon etc. and not be an issue. I would never weld a hole closed and see no need to plug it. Cutting what ends up being half holes thru the existing holes in the sight and into the barrel is best done with a carbide 1/8 end mill, the assembly being held rigid and in line on a fixture of some sort and in a good drill press or better yet a milling machine. Ream the holes right to left with a 2/0 taper reamer to a fit for the pins. By the time you get good clean holes reamed thru, you may need to go to oversize taper pins. You need to figure out some way to clamp the sight to the barrel so it does not move during the drill/ream operation (very important!) and some way to make sure the cutter comes out centered passing thru the bottom side of the sight as you drill thru. (Also very important!)
All this is way too much effort for a one time deal, especially if you do not have all the tooling, but I guess some things are done just because one wants to! Good luck.
                                                                               Pete

 I got the extention timed perfect, but my barrel setback I think is going to be an issue. I'm going to get a couple more extentions later and turn the barrel shoulder down to get the right headspace.

I know it is to tight because I had to modify my 22LR conversion to make it fit correctly.
The headspace I know is going to be way to tight, but for 22LR it should be great. Dry ran some 22LR in it and everything functioned like it should. I will test fire it next week.

Pete,
Thanks for that response again that was the information I was looking for.  

Tweak,
take note of how to do things the right way.