Upper parts selection guide

Barrel twist rate: 1:8 twist is the most versatile — it stabilizes everything from 55gr to 77gr bullets reliably. 1:7 is the mil-spec standard, optimized for longer, heavier projectiles and tracer rounds. 1:9 works for lighter bullets but struggles to stabilize 75gr and 77gr OTM bullets reliably.

Gas system length: Pistol-length gas on barrels under 10″. Carbine-length on 10–14″ barrels. Mid-length on 14–18″ barrels. Rifle-length on 18″ and longer. Mid-length on a 16″ barrel gives you longer dwell time, slower bolt carrier velocity, and a softer cycling impulse than carbine-length. The difference is noticeable, especially over a long range session.

Handguard fit: M-LOK is the current standard. KeyMod still works but M-LOK won the format war for new accessories. Free-float handguards improve accuracy by removing contact between the handguard and the barrel. Drop-in handguards are cheaper but barrel-contact can affect point of impact when pressure is applied.