AR-15 Parts List: Every Part and What Has to Match

Three barrel specs that matter

• Length: handling vs. velocity
• Twist rate: 1:7 handles heavier bullets; 1:8 is a solid all-around 5.56/.223 twist
• Gas system length: pistol, carbine, mid-length or rifle

Common gas lengths

• Pistol: very short barrels (around 7.5″) and most .300 BLK
• Carbine: about 10″ to 16″ barrels
• Mid-length: 14.5″ to 18″; softer on a 16″
• Rifle: 18″ and up

In-stock uppers and barrels ship in 2–3 business days. Every One Horse M-LOK handguard includes a steel barrel nut.

Usually the same

• Lower receiver and LPK
• Stock, grip and buffer tube
• BCG (same bolt face)
• Upper receiver and handguard

Not the same

• Barrel
• Muzzle threads: usually 1/2-28 vs 5/8-24
• Gas length: .300 BLK usually runs pistol gas, even on 16″ barrels
• Buffer tuning, especially subsonic .300 BLK

Safety warning: a .300 BLK round can chamber in a 5.56/.223 rifle and destroy it. Mark your magazines, mark your uppers and keep the ammo separate.

The 7 most common mistakes

1. Wrong gas tube length. Carbine tube, mid-length barrel. It won’t cycle.
2. Gas block doesn’t match the barrel journal. Gas leaks and the rifle short-strokes.
3. Commercial stock on a mil-spec tube (or the reverse). It wobbles or won’t go on.
4. Light buffer on a short, gassy barrel. Hard on the rifle and your shoulder.
5. Wrong muzzle thread pitch. A 5/8-24 device won’t go on a 1/2-28 barrel.
6. A “complete upper” with no BCG or charging handle. Read the listing. Ours include both.
7. Mixing .300 BLK and 5.56/.223 ammo. See the warning above.