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Thread adapter problem

Nwcid
I have an Armalite AR-30 in .300 Win Mag. The gun has the 26" barrel the the thread on muzzle break. The thread pitch is 5/8x18 and is 1.5" long since it uses a set nut for the break.

I have a .30 cal AWC Thundertrap that is threaded with the "standard" 5/8x24 thread pitch. I had a local gunsmith make me an adapter. He took some measurements of the gun and the suppressor and made me an adapter based on that. While it was close I ended up with an end cap (in the exit hole) strike where the bullet just barely grazed the side of the hole.

Next he took the barrel threading the inside then threading the outside while mounted on the gun. We took it out and shot it and the same thing happened. This guy has done a fair amount of other work (including threading) for me with great results. He says he is not sure why or what to do about it. Any thoughts?

I have considered having the barrel cut and threaded to match the can. I would like to keep the gun stock just because. If I had it cut and rethreaded I would have to cut off 1.5" of barrel which I am not real thrilled about.

Any thoughts or suggestions would be great. I really want to be able to use my can with this gun.
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Comments

8 comments

Date Votes
  • perry shooter
    I would think the first thing to do would to get a Range rod for the bore that would allow for the rod to be down thw muzzel end about 6 inches and then at the same time stick beyond the beyond the Can by about one inch with the range rod down the barrel thread the can on and make sure it has the same clearance 360 degrees at the end of the can this will tell you if you will have a strike when firing before finding out the hard way.
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  • rufe-snow
    My feeling is that there is a concentricity problem with the Armalite barrel, i.e. the axis of the bore isn't in the exact center. He's going to have to indicate I.D. of the bore with a tight fitting rod, while it's set up in the lathe.

    If it's not concentric to the O.D. of the barrel your going to have trouble getting the suppressor to work correctly regardless. The thread adapter isn't the problem, it's the bubbas who drilled and rifled the barrel.

    I would contact Armalite if it is a concentricity problem. Otherwise the only way I can see to fix this problem is to machine the adapter to correct the concentricity problem, i.e. the axis of the bore and suppressor are the same.
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  • v35
    You need to satisfy two issues; the thread on the AR needs to be concentric with the bore and all threads need to be class 3 threads.
    That means snug fit while threading on adaptor or can.
    Threads for your purposes need to be lathe cut (not die cut) with the bore running true not, just the barrel outside diameter.
    Threading an adaptor while mounted on the barrel and having concentricy problems tells me the bore isn't running true with the barrel outside diameter.
    You may have to remove the barrel and rethread it between centers.
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  • Nwcid
    The threads were cut on a lathe. As I said he has done other jobs for me including threading without problems. These were direct to barrel threads though. He too is a silencer owner so I am assuming (might be where my problem lays) he was doing it between centers.

    I know just enough about machining to think it is on par with magic [:)] While I do understand the concept I have not had the hands on to actually see how it work.

    I am not sure what class 3 threads are but the threads on the AR 30 are very wide pipe looking thread compared to all my other threaded stuff. I just assumed this had to do with the 18 TPI vs the 24 and 28 TPI I am use to. Would a pic help, if so I could get one up tomorrow.
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  • Tailgunner1954
    Nwcid

    Thread class has to do with the clearances and tolerances between the male and female components, not the pitch.
    He needs to make your threads concentric and parallel to the bore within .001 TIR (having the bore and the OD off by .020 TIR from the factory is not uncommon). Than, without braking down the lathe, the adapter needs to be tightly installed on your barrel and THAN you cut the threads on it to match your can.
    I'd suggest having all contact surfaces cut square to the barrel, crown, thread shoulder, lock nut, adapter inner and outer shoulders and the same surfaces on the can, so that the lockup surfaces as well as the threads are working to keep everything in line with the bore.
    Finaly, there is a reamer that is used with muzzle brakes, that pilots on the bore to insure that the brake/can bore is inline with the rifle bore
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  • rufe-snow
    If the barrel is readily removable from the receiver have him chuck it up in the lathe. Using a tight fitting rod indicate the axis of the bore. Compare this with the indicator reading of the barrel OD. If there is a great deal of runout between the two indicator readings, I would stop and contact Armalight about this discrepancy.

    The flashhider they originally had mounted on the rifle probably had a large enough I.D. that bore concentricy wasn't a issue, as it would be with a suppressor. Either that or the original 5/8 x 18 thread for the flashhider wasn't cut correctly, causing the misalignment.
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  • cpermd
    Sounds to me like he indexed off the barrel and not the bore.

    CP
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  • Nwcid
    Thanks for the input guys. I also got a couple of good emails too.

    I think I will start by calling Armalite and see what they have to say.
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