Muzzle Thread Pitch by Caliber: A Quick Reference

Before you buy a muzzle brake, flash hider, thread protector or adapter, you need to know two things about your barrel: its thread pitch and its caliber. This reference covers the thread pitches most often found on rifles and pistols sold in the US, how to read the notation, and how to check your own barrel.

How to read thread pitch

Muzzle threads are written as a diameter and a pitch.

• Imperial (inch) threads are written as outside diameter in inches by threads per inch (TPI). 1/2x28 means a 1/2 inch diameter with 28 threads per inch.
• Metric threads are written as outside diameter in millimeters by the distance between threads in millimeters, usually with an "M" in front. M13.5x1 means a 13.5 mm diameter with 1 mm between threads.

Two threads can share a diameter and still not fit. A 1/2x28 device will not thread onto a 1/2x36 barrel, even though both are 1/2 inch in diameter.

Right-hand vs. left-hand threads

Most muzzle threads are right-hand (RH): the device tightens clockwise, like a standard bolt. If no direction is given, right-hand is assumed. Left-hand (LH) threads tighten counterclockwise.

Left-hand threads are common on GLOCK, HK and SIG Sauer pistol barrels, and on 7.62x39 AKM-pattern rifles. A left-hand device will not start on a right-hand barrel of the same size, so check the direction as carefully as the size and pitch. Thread adapters can convert between them, for example from M13.5x1 LH to 1/2x28. GLOCK itself offers a 1/2x28 thread adapter for its threaded barrels.

Common thread pitches by caliber

These are common pairings published by firearm, suppressor and barrel makers and by major retailers. They are typical, not universal. Always confirm your specific barrel.

Notes on specific platforms

• GLOCK: GLOCK lists M13.5x1 LH for its 9mm threaded barrels and M16x1 LH for .45 Auto. Aftermarket GLOCK-pattern barrels are often threaded 1/2x28 instead, so check the barrel, not just the pistol model.
• SIG Sauer: Most 9mm SIG barrels are M13.5x1 LH, but Silencer Shop notes that some P320 barrels made in New Hampshire are 1/2x28.
• AK rifles: 7.62x39 AKM-pattern rifles typically use M14x1 LH. AK-74 pattern rifles use M24x1.5 RH, and on many of them the threads are on the front sight block rather than the barrel itself. Some AK models in 5.56 use 1/2x28 RH. Palmetto State Armory's own AK lineup includes models in all three.
• .30 caliber rifles: 5/8x24 is common, but some rifles use metric threads, such as M18x1 on some Sako and HK models and M18x1.5 on Accuracy International rifles.

Why caliber matters, not just threads

Thread pitch tells you whether a device will screw on. It does not tell you whether the device is safe for your cartridge. Every muzzle device has its own bore, sized for a caliber. The same 5/8x24 thread is used on brakes bored for 6.5mm and on brakes bored for .30 caliber. A brake bored for 6.5mm should not be used on a .30 caliber rifle, even though it will thread on.

• Use a device only with the calibers its maker rates it for.
• Do not fire a cartridge larger than the device's rated caliber through it, even if the threads match.
• A 1/2x28 thread is common to both .22 LR and 5.56, and a 5/8x24 thread to .300 Blackout, 6.5 Creedmoor and .308. Check the caliber marking on the device every time you move it between guns.

How to check your barrel

1. Unload the firearm. Remove the magazine and ammunition and visually confirm the chamber is empty.
2. Check the documentation. The owner's manual, the barrel maker's website or the original product listing usually states the thread pitch. Barrel, suppressor and device makers all recommend confirming with the manufacturer before ordering.
3. Measure the diameter. Use calipers across the outside of the threads. Threads are often cut slightly undersized, so a 1/2 inch thread may measure a little under 0.500 in.
4. Measure the pitch. A thread pitch gauge, available in imperial and metric sets, is the most reliable method. Without one, count thread peaks over a half inch and double it for TPI. For metric, measure across several peaks and divide by the number of peaks.
5. Check the direction. Look at the angle of the threads, or test with a known device by hand. Never force a device that does not start smoothly.

When installing a device, follow the maker's instructions for timing and torque. Brakes and flash hiders are usually timed with a crush washer or shims, and suppressor owners often check bore alignment with an alignment rod before first use.

Shop

Browse muzzle devices by caliber and thread pitch, cover exposed threads with thread protectors, or convert between pitches with thread adapters and shims. Each listing states its thread pitch and rated caliber.