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03-17-2012, 12:00 AM #1
Threading an AR 15 barrel for a new flash suppressor
A friend of mine wants to know if I could tap new threads into on the end of his AR barrel to acomodate a new flash suppressor.
He has one installed that is held on by just one small allen set screw. It has actually come off while firing it. So since he knows I own a lathe(South Bend Heavy 10) and a mill(Bridgport), he has asked me if I could do this for him. I have owned these machines for couple of years now and cosider myself a novice machinist and can hold a pretty tight tolerance. I do some type of maching almost every day but by no means do I consider myself a professional, not by a long shot ...at least not yet ... maybe someday.
First, I know the through hole on my heavt 10 spindle is 1 3/8 but will an AR barrel that is separated from the upper reciever fit into the spindle?
Is the AR barrel a press fit or is it screwed onto the upper reciever?
Is it neccessary to separate the barrel from the upper reciever?
Is it neccessary to build a cathead?
Depending on which type of flash suppressor he gets, must the barrel be turned down to a smaller dim. to begin the threading operation?
I guess these questions are enough for now. I know that you guys on here are real machinist's and gunsmith's so I look forward to getting some good feed back from you.
03-17-2012, 07:30 AM #2
With the barrel remove from the receiver, it should fit through your headstock. The barrel is held in the receiver by a special nut located at the rear of the hand guard. You'll need to buy or make a wrench to remove it.
Mil-Spec Armorer's Barrel Wrench AR-15 Steel
Most AR barrels are threaded 1/2-28 about 0.6" in length. While concentricity isn't absolutely critical for a flash suppressor, if there is any chance a suppressor might be used on the gun in the future, you should indicator off the bore before cutting the threads.
03-17-2012, 07:50 AM #3
Take the barrel off the upper reciever. Set it up in your lathe between centers. The tread should be 1/2-28 .600 long. This is the only way a barrel should be threaded to make sure the threads are concentric to the bore.
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03-17-2012, 11:28 AM #4
Does the gas tube need to come off? And if so, does it come off easily? I'm imagining that the barrel with the gas tube still on it can fit and can be placed between the jaws of a 4-jaw chuck so that I can get it concentric ... am I correct?
03-17-2012, 12:03 PM #5
I'm pretty sure you need to turn this between centers that are located on the bore. This is the only way to make sure you get it on the same axis. If you just indicate the end you are not sure you have it true to the entire length. You might be able to put a mandrel in the bore and indicate that to check the axial alignment.
03-17-2012, 12:20 PM #6
To do the job correctly you will need to remove the Front sight / gas block, the gas tube is pinned to it and will come off as 1 piece. Then you need to remove the Delta Ring assem, assuming he has standard handguards. If he has a Free Flaot System unscrew the rails or tube and then unscrew the barrel nut. Once you do that the barrel will come out of the reciever. You will need to turn the barrel between centers so that the threads are concentric to the bore. Using a die or just clamping the barrel in a 4 jaw will NOT work properly and you will get a non concentric thread. A non concentric thread could cause a strike on the break or if he or a future owner decides to put a silencer on your non conctric thread they will have grenade on the end of the barrel. Baffle strikes are a serious problem with barrels that people thread without doing it correctly. Between centers is the only method I would recommend if you want a quality result.
Page 6 should give you everything you need to do the job to spec. http://www.silencertalk.com/AAC-thread-guide.pdf
On a side note you should have a vise block to hold the upper while uninstalling and reinstilling the barrel. Some people use soft wooden jaws with mixed result that sometime result in damage to the finish.
03-17-2012, 04:18 PM #7
03-17-2012, 04:33 PM #8
Hey MrM4, I found it on Brownell's website. Thanks again.
If it's OK, I'd like to post pics of this process on here. I know that this probably not such a big deal for all of you experianced gunsmiths but it will be a big deal to me as this will be my first gunsmithing machining operation.
03-17-2012, 07:51 PM #9
You do not need to remove the front sight tower. Remove the handguards and drive out the roll pin in the sight tower that captivates the gas tube. Remove the gas tube and separate the barrel from the receiver. You can clamp the barrel in a padded vise and use the AR tool or use the plastic AR upper receiver vise insert and the AR tool to remove the barrel. Mount the barrel between centers on your lathe and be mindful of the sight tower (use your lathe dog to offset the sight tower) and thread as mentioned above. AR's with free float handguards are a bit different but similar.
03-17-2012, 08:07 PM #10
Doing the 1/2-28 thread between centers is the right way to do it. If you're bud's already got a BBL with a suppressor, though, there should be threads on there which might just need to be cleaned up.
Turning an AR barrel between centers will be tricky, though. The barrel extension MUSTN'T be removed from an AR barrel. You'll need to make a dead center for the headstock side that is skinny [probably about 3/8" diameter] and protrudes a good inch or so before tapering to the center point.
Don't forget to recrown the barrel after turning it between centers. Crowning is easy to do, but a necessary step that many neglect.
03-17-2012, 08:09 PM #11
Leaving the gas-block/sight tower in place presents a safety hazard when working on the lathe. That's just asking to grab the sleeve of your shirt...
03-17-2012, 08:15 PM #12
If you insist on turning between centers, you better plan on recrowning afterwards. [Oops, paramedico beat me to it] You also better be sure that your tailstock is aligned properly.
Personally I see no reason to go that route provided you indicate off the bore (with an indicator rod) and hold the barrel reasonably well centered where it exits the headstop. i.e. use a spider or something similar. I've threaded a good number of barrels this way and have yet to have any concentricity problems or baffle strikes.
03-18-2012, 01:56 PM #13
03-18-2012, 02:59 PM #14
Brownells sells those little brass gizmos as well. Certainly be easy enough to make your own too.
As far as the not removing front sight tower goes, remember that guns are dangerous to begin with. And the sight tower is certainly no more dangerous than a lathe dog spinning around. You can set your center rest up to protect your loose sleeve (that you shouldn't be wearing) from catching on it too. Not all sight towers come off easily. I have probably removed a dozen or so and at least half of them had to have the pins drilled on the small end to get them out. I think they sometimes get peened over at the factory and then there is no way but to drill. Then there is the use of hard drill resistant pins. And yes, I am aware that the pins are tapered.
03-18-2012, 04:21 PM #15
Gazz has a point you could leave the Front sight / gas block on the barrel and do it his way and it would work. I choose to remove them when I do a barrel, it is not a "must be done this way" type thing. Some times they come off hard but with the right tools I've never had to much trouble. As for recrowning, if you damage the crown then yes it will need to be recut.
If you have a machine that allows you to use a spider on the back end of the head stock thats fine if you have a indicator rod of the right size. I would not trust the OD of the barrel extension to be concentric with the bore. There are way to many "home build" ARs out there to trust anything but the actual bore in my opinion. If I am going to put a $1200 Titanium Silencer on the end of the barrel I want it as close to perfect as I can get, however for the stated use of a Muzzle Break there may be more tolerance for error due to a shorter OAL of the Break compared to a silencer and often a larger ID threw the part. That being said guys sell uppers all the time I would hate be the guy who buys one and finds out the thread job was less then perfect.
03-18-2012, 04:53 PM #16
so when you are turning between centers, how are you 100% positive the crown is dead nuts aligned to the bore? when turning between centers, how do you know the bore is any more concentric an inch or two in from the crown than it would be by directly indicating off the bore while holding the od in a 4 jaw?
03-18-2012, 05:00 PM #17
I would also double check CA law and whether or not his rifle would be deemed too evil for civilians and cause cancer to babies if the barrel were threaded.
Last edited by kuraki556; 03-18-2012 at 05:00 PM. Reason: typo
03-18-2012, 11:42 PM #18
03-19-2012, 11:41 AM #19