What thread pitch does my barrel need?

The short answer

For most rifles it is 1/2×28 under .30 caliber and 5/8×24 at .30 caliber — but the thread is the least interesting part of the job, and picking it is not the same as cutting it well.

Why bore diameter decides the thread

A muzzle thread has to be cut into steel that is still thick enough to hold it. The larger the hole down the middle, the less material is left at a given outside diameter — so the thread has to grow with the bore. That is the whole logic, and it is why the standards cluster the way they do.

The corollary matters more than the rule: a thread that is correct for your caliber may still be wrong for your particular barrel if the muzzle has been turned down thin, contoured, or previously threaded. That is a measurement, not a lookup, and it is the first thing we check.

The standards you will actually encounter

1/2×28 is the common thread for .223 and 5.56 rifles, and for .22 LR. It is also frequently found on 9mm pistol barrels in the United States, though 1/2×36 appears on 9mm often enough that it should never be assumed.

5/8×24 is the standard at .30 caliber — .308 Winchester, .300 Blackout, 6.5 Creedmoor and their neighbours. It is the second thread most people meet, and the one most commonly needed when someone moves up from an AR-15 to a precision rifle.

.578×28 is the usual thread for .45 ACP pistol barrels. Larger magnum rifle calibers move to 3/4×24 and above. Several pistols, particularly European designs, use metric threads such as M13.5×1 LH and M14.5×1 LH — and the left-hand designation on those is not optional trivia.

Concentricity is the part you are paying for

A suppressor mounts on that thread and puts several inches of tube, and a good deal of mass, in front of your muzzle. If the thread is not concentric to the bore, the bullet does not exit down the centre of that tube. Best case, your groups open up. Worst case, the projectile strikes the baffle stack and destroys a suppressor that costs considerably more than the barrel.

Cutting a thread true to the bore means indicating the barrel in on its own axis before any metal comes off — not chucking it by the outside diameter and trusting that the two agree, because on a great many barrels they do not. It is slower, it is the reason the job has a real price, and it is not visible in a photograph of the finished thread.

What comes off the lathe

A completed job is threaded to specification, crowned, and shipped with a thread protector fitted. The crown matters for the same reason the concentricity does: it is the last surface the bullet touches, and a damaged or off-axis crown will undo accuracy work done anywhere else on the rifle.

If you are threading in order to run a suppressor you do not yet own, tell us — the item you intend to mount can determine whether a shoulder needs to be cut a particular way, and that is much cheaper to get right the first time.

Barrel Threading at Lemcoe Precision →