The More You Know: Thread Pattern Basics

Credit: http://sub-silentsuppressors.com/

Credit: http://sub-silentsuppressors.com/

That sharp, corkscrew section on the end of your barrel is called the ‘threads’. And it is completely ok to admit that is where your level of knowledge on barrel threading ends (until recently, I was right there with you). For those of us who run stock barrels and muzzle devices, you probably have never had the need to get into the weeds when it comes to thread specifications. But some mounting options remind us that not all barrel threads are created equal – a fact that is especially true when it comes sound suppressors.

Most gun/upper/barrel manufacturers should print their thread specifications either on the owners manual or on the product page of their website. And some muzzle devices actually have the thread infomation stamped or engraved directly on the mount itself. But what if you have an older rifle or an imported pistol where you are forced to do a bit of research before buying a new piston or mount?

While your best bet is to take your gun to a qualified gunsmith, most of us like to a lot of the maintenance and customization ourselves. Luckily, there are some basic techniques you can use to find out more about your barrels threading without having to see a specialist.

Standard vs Metric:

  • Most US made components are threaded to US/English/Standard specifications. These threads are measured in Threads Per Inch (TPI).
  • Most foreign components are threaded to Metric specifications. These threads are measured in pitch, which is the distance between adjacent threads.

Both types of threads can also come in either left or right handed twist, but in the barrel-world, Metric specs are more likely to be a different “handedness”.

From the Screw Thread Wikipedia page:

The helix of a thread can twist in two possible directions, which is known as handedness. Most threads are oriented so that the threaded item, when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a clockwise direction, and moves towards the viewer when it is turned counterclockwise. This is known as a right-handed (RH) thread, because it follows the right hand grip rule. Threads oriented in the opposite direction are known as left-handed (LH).

By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads.

image

Rotating rockets are cool!

Some shooters prefer mounts that come in left-handed threading. The theory is that with the rotation of the bullet in the same direction of the threads, the mount might have a tendency to “walk” or loosen.

Here is basic diagram of thread attributes:

 

Credit: http://specbolt.com/technical.html

Credit: http://specbolt.com/technical.html

 

Here are two examples of common barrel threads:

US/Standard/English


1/2″x28

1/2″ – Outside Diameter

28 = Threads per inch

Credit: https://www.boltdepot.com

Credit: https://www.boltdepot.com

 

Metric


M13.5×1 RH

13.5mm – Outside Diameter

1 – Thread Pitch

RH – Right Handed Twist

threads-pitch

Credit: https://www.boltdepot.com

 

Silencer Shop has a good write-up on certain firearms and their thread patterns here: http://blog.silencershop.com/common-barrel-thread-reference/

I’ve summarized the basics:

.22 LR

  • Standard 1/2×28

9mm

  • Standard 1/2×28
  • Standard 1/2×36 – Some 9mm carbines
  • M13.5×1 LH – H&K, Sig Sauer

.40 S&W

  • Standard: 9/16×24
  • M14.5×1 LH – H&K, Sig Sauer

.45

  • Standard: .578×28
  • M16x1 LH – H&K USP Tactical uses the
  • M16x1 RH – H&K Mark 23 (Socom)

5.56mm (.223)

  • Standard: 1/2×28
  • M13x1 LH – Steyr Aug
  • M15x1- H&K
  • 9/16×24 – Ruger Mini

7.62mm (.308)/.338LM

  • Standard: 5/8×24
  • M18x1 – Sako TRG
  • M18x1.5 – Accuracy International
  • M14x1LH – Some AK47 patterned rifles.
  • 3/4×24 – Desert Tactical Arms

Why all the different sizes for different calibers? Because the differences in bore diameter will change the amount of leftover barrel material (sometimes referred to as ‘meat’) for threading. Smaller bores can have smaller threads.

There are also several YouTube videos on barrel threads. Here’s a couple to review:

 

And just to complicate everything a little bit more, some suppressor manufacturers call for their own subset of barrel threading specifications. They should provide schematics that a machinist can use to cut a barrel correctly. As an example, SilencerCo has two different thread patterns for 1/2×28 – one for rimfire silencers and one for centerfire rifle silencers. Their rimfire suppressors use an o-ring engagement surface that needs a special cut to take full advantage of the mount.

image image

Also, another important distinction: 1/2″x28 rimfire threading should be cut to a total length of 0.4″ whereas 1/2″x28 centerfire should be cut to a total length if 0.6″ – a fact that is missed by some barrel manufacturers and machinists. This can cause some suppressors to index/seat on the blast baffle rather than the barrel shoulder.

Final thoughts: just because your barrel is threaded, doesn’t meant that it is ready to spin on a silencer. Some barrels, the AK variants for instance, are notorious for being out of spec. While this is usually ok for a flash hider or muzzle brake, using inadequate threading with a sound suppressor is asking for trouble.

Barrel Threading Basics – And that’s The More You Know

tmyk



Pete

LE – Science – OSINT.
On a mission to make all of my guns as quiet as possible.
Pete.M@staff.thefirearmblog.com


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  • Kasper

    I have never seen any left handed/links (German for left) threads on any barrel. At all. Metric or not.

    The thing about the device ‘walking’ off is nicely fixed by, for instance the Sonic suppressors, where the flash hider part, has twisting outlets, that uses the escaping gasses to turn the suppressor on tighter. Handedness is not an issue as long at the threads match.

    All barrel end devices should preferably rest against the face(I don’t know the correct term in English, but where the threads stop and the barrel regains it original diameter) and not depend on the thread for providing perpendicularity to the bore. I know timing may sometime need a lock nut – that is not optimal, but probably acceptable.

    • JD

      nearly all of HK’s pistol threading is left handed

    • PK

      The majority of my pistols are LH threaded, in 13x1mm and 14x1mm. I find it more convenient since I’m usually holding them pointed away from me… LH, away from me, seems very much a case of “turn RH to tighten”.

      The “face” you’re looking for has a no-less-confusing name of “shoulder”. I agree, the threads simply hold the muzzle device, whatever it may be, against the shoulder tightly, while that surface is what truly indexes things in place. Concentricity is king, and in the case of that shoulder, nearly all that matters. As long as the threads are concentric to the direction of the bore, and the shoulder is perpendicular, things are good.

      Lock nuts aren’t the best thing in these situations, but as you note they are sometimes needed. I find it better when the design was made with such a device in mind, as with the Salvo shotgun silencer.

    • BrandonAKsALot

      Every Kalashnikov based rifle designed before the 74 had left-handed threads and many after that. I believe FN cuts LH threads on the F2000 also. There are plenty, but majority produced these days are RH.

  • Exoskeleton

    Hi Pete, very interesting write up, thanks!

    I have been delving into the same subject the last week or so. The main question I have is, which thread type is better for consistency, metric or Unified Screw Thread Standard Series?

    Another thing to note is that there are different classes for Unified Screw Thread Standard Series threads that refers to the type of fit or tolerance between the internal and external thread. A class 1 fit is loose while a class 3 is tight. An “A” refers to the external thread while a “B” refers to the internal thread. Check the end of the thread specification for class specification, for example .500-28 UNEF-3A.

    Cheers

    • Pete – TFB Writer

      Good question: I’d guess the finer threads, whether they are US or Metric, would result in better consistency.

      Great info on UNEF. I’ll add it to the post later today. Thank you.

      • PK

        If you’re modifying the post…

        “M14x1LH – Some AK47 patterned rifles”

        Scratch that. Almost all, but don’t trust them to be concentric, and since the muzzle device indexes on a pin and the front sight post… well, Russian silencers are oversized bore for a reason.

        How about AK74 rifles? That comparatively huge thread is 24×1.5mm, but RH instead of LH.

    • PK

      There are a lot of factors to consider, but I’ll try my best to sum up.

      Your guess on 3A/B is correct with regards to consistent POI/POA shift with a device on/off. The other thing that matters quite a bit is total thread engagement, bearing surface on the shoulder/barrel/mount and how well that was done, is everything concentric, are the threads standard 60° V, are they 55°, are the buttress, are they acme….

      In general, 3A/B binds too fast the moment things get dirty. Your muzzle device is now stuck. 1A/B is too loose, you’ve got an area saturation barrel now. 2A/2B, for direct-thread items which come on/off regularly, is just right – so long as you have a very perpendicular, concentric shoulder to butt against the same way every single time. If not, 3A/3B is a better choice.

      • Pete – TFB Writer

        Awesome. Thank you PK.

        • PK

          No problem! I feel like I’m still learning some of this, and I’ve been threading barrels for a while, now. I’ve been cutting threads in metal for a whole lot longer, too, and there are a lot of tricks to know.

          What it boils down to is that there’s a reason gunsmiths charge what they do for threading. It’s basically never a “grab a tap and use it on the muzzle” type of job unless you don’t care about ever using a silencer, for example. Concentricity, and that shoulder/device interface, really do matter… but all I ever hear people concerned about (except for enthusiasts) is the thread quality itself.

      • Exoskeleton

        Thanks for the info PK!

  • Tim

    “Some shooters prefer mounts that come in left-handed threading. The theory is that with the rotation of the bullet in the same direction of the threads, the mount might have a tendency to “walk” or loosen.”
    A question I’ve been wondering about for some time, especially after reading about new ‘fluted’ self defense bullets, is there a standard or custom that barrel rifling is all right handed, as the quote above implies??

  • Tim

    “Some shooters prefer mounts that come in left-handed threading. The theory is that with the rotation of the bullet in the same direction of the threads, the mount might have a tendency to “walk” or loosen.”
    A question I’ve been wondering about for some time, especially after reading about new ‘fluted’ self defense bullets, is there a standard or custom that barrel rifling is all right handed, as the quote above implies??

    • Pete – TFB Writer

      I had the same question as I was writing that, what percentage of rifled are right handed vs left handed. I need to research. Thanks

  • Old Gringo

    I have an AK that came with the 14.1mm left hand thread…..the problem is that millions are floating around out there from dozens of countries….for that reason, the standard rule is to put a dowel rod thru the suppressor when it is mounted to insure the threads are cut straight….otherwise, your $1,000 .30 call suppressor might catch the edge of a round going away at 2,300 fps….not a good idea…mine shoots fine…but you never now……there are suppressors made just for AK variants..the Huntertown Arms makes one.. called the AK Kestrel, to be safe the bore of the suppressors is actually .400…..they are not quiet, but do cut down the noise of the 7.62 round to “hearing safe”…and they kill the flash somewhat.. that big bore is there only to make sure the thread pattern angle does not result in an end strike….