M4A1 vs SIG Sauer 556xi

1994–Present

2010–2019

~10,000,000+ units (including all variants and licensed production)

Approximately 20,000

838 mm (33.0 in) stock extended, 756 mm (29.8 in) stock collapsed

914 mm

368 mm (14.5 in) standard; 406 mm (16 in) for the M4A1 SOPMOD variant

419 mm

3.07 kg (6.8 lb) empty, 3.74 kg (8.2 lb) with full 30‑round magazine

3.5 kg

30 rounds standard STANAG, 20‑round low‑profile, 40‑round drum, 60‑round Beta‑C‑Mag available

30 rounds

Short‑stroke gas piston (M4A1 variant) with rotating bolt; original M4 uses direct‑impingement

Gas-operated, long-stroke piston

Two‑stage military trigger, full‑auto only selector on M4A1; semi‑auto on standard M4

Single-stage

Standard: A2 front post and rear flip‑up aperture; optional: MIL‑STD-1913 Picatinny rail for aftermarket optics, tritium night sights, or holographic sights

Flip-up rear, post front