mikr: With 7 grains weight difference? I think the steel penetrator is also a factor for M855.
7 grains is 13% difference in mass - a whole lot!
M193: Defined by: Mil-C-9963F 55 grain bullet at a velocity of 3,165 from a 20" barrel @ 78 feet from the muzzle.
It's ballistic coefficient is typically .243
M855: Defined in MIL-C-63989 a 61.7 grain at a velocity of 3,000 fps from a 20" barrel @ 78 feet from the muzzle.
It's ballistic coefficient is typically .304.
Kinetic Energy of a bullet is M * V * V which is mass multiplied by square of velocity.
At 78 feet from 20" barrel:
M193: (55 * 3,165 * 3,165) / 1,000,000 = 550.95 units
M855: (62 * 3,000 * 3,000) / 1,000,000 = 558.00 units
M855 has 1.3% higher energy is beacuse square of velocities differ by 'only' 11% which is less than 13% difference in mass.
So, despite being slower, M855 has slightly (1.3%) higher energy because 11% deficit in (square of) veocity is compensated by 13% extra mass. More of 855's energy is due to mass rather than velocity compared to faster m193.
On top of that M855 retains energy better due to:
1. better ballistic coefficient (being longer)
2. slower speed - since air resistance is roughly proportional to the square of the velocity - the faster it goes, the faster it loses energy over the same distance! After certain distance (4-500 yards?) M855 will not only have way more energy but even more velocity than m193!
Why would anyone shoot lighter ammo then? Because recoil is proportional to M * V, not V square and trajectory of faster bullet flatter - less bullet drop.
If you expect to shoot at closer range (0-200 yards), having more controllable weapon and less sensitivity to aiming errors as well as lighter burden (or more ammo you can carry) of is more important than extra energy.
I think the steel penetrator is also a factor...
Ballistics-wise steel penetrator being lighter than led shifts the bullet's center of ballance backward and makes it less stable in the air. That necessitates at least 1/9" spin to balance the bullet. Older M16s with 1/12" can shoot M193 no problem but cannot shoot M855 accurately even at 100 yards - especially in colder air. The bullet starts to tumble after it leaves the barrel.
Supposedely heavier steel-tipped M855 is better against flack-jacketed troops at longer distances than M193.
Here goes my first post!