The disparity between 5.56x45mm cartridges loaded with 55-grain projectiles and those with 62-grain projectiles stems primarily from the projectile weight. This seemingly small difference affects several ballistic characteristics. A lighter, 55-grain bullet typically achieves a higher muzzle velocity, leading to a flatter trajectory at shorter ranges and increased fragmentation upon impact. Conversely, the heavier, 62-grain bullet, while possessing lower muzzle velocity, retains more energy and velocity downrange, offering better performance at longer distances and improved penetration against barriers.
Understanding this distinction is crucial for selecting the appropriate ammunition for specific needs. Military and law enforcement may favor heavier projectiles for their enhanced penetration and terminal ballistics, particularly in urban environments or when engaging targets behind cover. Civilians participating in competitions or hunting might opt for lighter bullets for their flatter trajectory and reduced recoil, especially at shorter ranges. The choice between these two projectile weights involves a trade-off between velocity, energy, and trajectory.