There are many standard thread pitches and some odd ducks out there. We’ll review the most common thread pitches, sometimes shown as “TPI” (Threads Per Inch) for standard US threads. Sometimes metric thread pitches are used in barrel threading. We’ll cover both.
Why does it matter?
In order of size, the most common US threads used on rifles are 1/2x28, 9/16x24, and 5/16x24. This is very important as you consider what you plan to be attaching to your threaded barrel. If you are considering a suppressor or other muzzle device, for example, you will want to know your thread pitch (and bore diameter/caliber) before you order.
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Standard US thread pitches
1/2x28 is common to most .22 caliber bores. A standard AR15 chambered in .223/5.56 usually has a muzzle threaded to 1/2x28 and many 22LR’s are as well like a Ruger 10-22, Ruger Charger, and Ruger 10-22 Takedown. The small bore of a .22 cal leaves enough wall thickness (material between the bore and outside barrel diameter or “OD”) to accommodate a 1/2 inch thread. 1/2x28 barrel threading is not ideal for .30 cal bores (although sometimes used) as it leaves a fairly thin barrel wall after it is threaded (.30 hole and a .50 OD don’t leave much - especially when you consider the cut thread depth makes the structural OD even thinner) Occasionally you will see a 30 cal barrel threaded to 1/2x32 to avoid the problem of threading a 1/2x28 .22cal muzzle device on a .30 cal bore. See the problem? a .30 cal projectile leaving a barrel doesn’t fit through a .22 cal muzzle device and is a very dangerous combo. If you are trying to attach a 1/2” device and find the threads are binding, stop immediately and take a closer look, you may have a dangerous mis-match.
9/16x24 is more often used for .30 cal rifles chambered in .308, 300 win mag, 30-06, 300 WSM etc. This allows more wall thickness and works very well on platforms like the AR-10, FAL, and M1A or M14.
5/8x24 is similar to 9/16x24 barrel threads - just bigger. 5/8x24 can is often used for a .30 cal bore or larger. This is sometimes used when the barrel being threaded has a big enough OD to accommodate the larger thread size. Many times, we thread .308 AR bull barrels in 5/16x24. If you got the OD, why not use it?
Metric Thread Pitches
14x1 LH METRIC is a common thread on AK variants. This thread pitch is very close on dimensionally to 9/16x24, which makes sense as an AK in 7.62x39 has a .30 cal bore roughly.
What about this RH (Right Hand) or LH (Left Hand) business?
If you have ever removed a left hand thread you should have noticed right away. Yep, it’s weird, but it has it’s place. For whatever reason (there are reasons including barrel twist direction), the communist countries often chose LH 14x1 metric threads. Some English barrels are threaded with left handed threads as well as can be found on some FAL’s with 9/16x24 LH threads. Typically, RH threads are assumed unless it is noted by “LH” as in 9/16x24 LH.
Any good suppressor dealer can advise you on a good match between a suppressor, your caliber, and thread pitch. Any competent gunsmith can as well. Be careful about asking “Bob” down at the gun & tackle shop…leave Bob to advise on what the fish are biting down at the lake.
There are other thread pitches not covered here. This is meant to give you an overview of the common threads often used in barrel threading. (on a side note: It may interest you to know, an UZI barrel nut is a whopping 1x18” thread - yes 1” diameter - and there are suppressors made to screw on in place of that barrel nut. Dang cool by the way.)
If you have any questions, feel free to contact us and we’ll see what we can do to help.
*You are responsible to seek professional advice before making any modifications to your firearm, or before making any decisions regarding your firearm. Be smart. Get expert advice. This article is therefore for informational purposes only. Always consult a qualified gunsmith in your area regarding any firearm modifications.