The SIG Sauer P320‑M18 3.9″ Slide Assembly is a factory‑built, optics‑ready M18 slide finished in black and delivered with SIG’s ROMEO‑M17 reflex sight and SIGLITE iron sights already installed. Built from stainless steel and listed at ≈12.4 oz, this 3.9″ 9mm slide assembly includes a loaded‑chamber indicator and the military‑style SIG‑LOC optic footprint for drop‑in compatibility with M17/M18‑pattern optics. Sold as a slide assembly (barrel and recoil components typically not included), it’s an ideal factory option for converting a compatible P320 fire control unit into an M18‑style pistol with a pre‑installed ROMEO‑M17 for out‑of‑the‑box sighting performance.