How to Choose Thread Pitch for 4-inch AR Pistols 

Quick answer (TL;DR)

For most 4-inch AR-style pistols chambered in 5.56/.223, the industry standard is 1/2?–28 thread pitch. If you run a .30-cal family (e.g., .300 AAC Blackout, .308 variants) you’ll most often see 5/8?–24. International and some OEM pistols/uppers use metric threads (for example M13.5×1 LH on some HK/SIG barrels), and adapters exist to convert between common sizes. Always verify your barrel manufacturer’s spec and measure before buying muzzle devices or suppressor mounts. 

Why thread pitch matters on a 4-inch AR pistol

Thread pitch determines whether a muzzle device screws on, indexes (times) correctly, seals for a suppressor, and survives repeated firing with short-barrel gas dynamics. On a short 4-inch barrel you have less barrel length to manage gas pressure and muzzle flash, so the muzzle device choice—and therefore the correct thread—directly affects reliability, recoil control, and shooter comfort in confined spaces. Don’t assume “one size fits all”; a mismatched device can be dangerous or at least harmful to accuracy and timing. 

Standard thread pitches by common calibers (quick reference)

• 5.56 NATO / .223 Rem. — 1/2?-28 (the de-facto AR-15 standard). 

• .300 AAC Blackout (.300 BLK) — usually 5/8?-24 (designed to head-space and perform well in short barrels / suppressed setups). 

• .308 / 7.62 family / AR-10 — 5/8?-24 is very common. 

• 9mm AR variants — commonly 1/2?-28 or 1/2?-36; some OEMs (HK, Sig) use M13.5×1 LH or other metric threads. Always confirm. 

Pro tip: if you own an AR-15 upper in 5.56/.223, your safest bet for compatible muzzle devices is 1/2-28. If you switch to .300 BLK (same upper/bolt but different barrel), expect 5/8-24 on many factory barrels. 

Why short barrels (4") change the decision

A 4-inch barrel has much higher gas pressure near the muzzle and produces a larger flash/pressure spike than a 14.5–16" barrel. That influences two thread-related choices:

1. Muzzle device selection: You’ll likely want a compensator/flash reducer designed for short-barrel use (different gas routing and timing). Those devices must be made for your caliber’s bore diameter even if thread pitch is the same (a 1/2-28 device for 5.56 has a smaller bore than a 1/2-28 device made for 9mm). 

2. Suppressor mounting & timing: If you intend to suppress, choose threads that match the suppressor or a robust adapter system; using an adapter introduces more parts that need correct torque and timing to avoid baffle strikes. Given short-barrel gas dynamics, proper gas tuning and device timing become even more critical. 

Measuring your thread (how to confirm before you buy)

1. Check manufacturer spec sheet: This is the simplest — barrel producers list thread pitch and direction. Always trust the manufacturer first. 

2. Use a caliper + pitch gauge: Measure major diameter (outer thread diameter) and verify TPI (threads per inch) with a thread gauge. If you’re not comfortable, take the barrel or muzzle device to a competent gunsmith or the shop you plan to order accessories from. 

3. Beware left-hand threads and OEM oddities: Some manufacturers (HK, FN, SIG) use left-hand or metric pitches on certain pistols/uppers — don’t assume right-hand 1/2-28. 

Metric threads & suppressed builds — what to watch for

If you plan to use a suppressor, metric threads are common on many pistol platforms (M13.5×1 LH is often used on some HK/SIG pistols). Some suppressors use quick-attach mounts that require a matching muzzle mount or adapter. Confirm whether your suppressor expects a right-hand or left-hand thread and whether the mount is indexed (timed) or uses an indexing shims/washers system. Double-checking these specs prevents baffle strikes. 

Adapters, timing, and indexing — practical solutions

• Adapters: If your barrel is 1/2-28 but your desired muzzle device is 5/8-24 (or vice-versa), you can use high-quality thread adapters — but add one more interface that must be torqued and timed correctly. Use military-grade adapters from reputable makers and verify heat treatment/coating for durability. 

• Indexing: Many flash hiders and muzzle brakes must be timed (so ports face up/out). Use crush washers, shims, or timed nut systems designed for your pitch. On short barrels where devices are critical to recoil control, take the extra time to index correctly. 

Practical checklist for choosing thread pitch on a 4-inch AR pistol

1. Identify your caliber and check the barrel spec sheet.

2. Measure with a caliper and TPI gauge if the spec is unavailable. 

3. Match bore diameter and device bore even when TPI matches (9mm vs 5.56 devices are NOT interchangeable despite shared 1/2-28 threads in some cases).

4. If using a suppressor, confirm thread direction (RH vs LH) and metric vs imperial pitch. 

5. If you need a different pitch, choose a high-quality adapter and test for timing and durability. 

Safety, legal, and final notes

• Always follow safe handling, storage, and local laws when building or modifying firearms. If you plan to use suppressors, comply with NFA rules in the U.S. (Form 4 / tax stamp) and local laws.

• When in doubt about thread specs or suppressor compatibility, consult the barrel manufacturer, a reputable gunsmith, or the suppressor vendor — mistakes can be expensive and dangerous. 

FAQ

Q: Can I use a 1/2-28 muzzle device on a 5/8-24 barrel with an adapter?
A: Yes — with a quality adapter, but adapters add weight, length, and another failure point. Test timing and torque. 

Q: Is 1/2-28 always the AR-15 standard?
A: For 5.56/.223 AR-15 barrels, yes — 1/2-28 is the standard; but exceptions and metric/OEM threads exist, so confirm the exact barrel.