The M249 SAW (Squad Automatic Weapon) is chonky. It’s a nightmare to clean. It’s a reasonably heavy, demanding, ammo jam waiting to happen, that will cycle the one speck of dust you missed at 3 a.m.

Somewhere between the clack of disintegrating links and the sharp bite of spent powder hanging in the air, the Army ran into a reality every grunt learns the hard way. “Good enough” dies fast when the ...