*At 50 meters, the 7.62mm ball round cannot reliably penetrate a single layer of well-packed sandbags. It can penetrate a single sandbag layer at 200 meters, but not a double layer. The armor-piercing round does only slightly better against sandbags. It cannot penetrate a double layer but can penetrate up to 10 inches at 600 meters.
The penetration of the 7.62mm round is best at 600 meters. Most urban targets are closer. The longest effective range is usually 200 meters or less. *
Wouldn’t the bullet be traveling the fastest (and therefore have the best penetration) when it exits the barrel? Why does the penetration go up with distance (until a certain point where it must drop off again)?
Remember the episode of Mythbusters when they shot bullets into the water? Presumably it penetrates more because it is going slower. The bullets are made for shooting people, not walls. If a bullet is going too fast as it hits a target, it gets damaged and can explode.
Over at The Box O’ Truth they’ve done lots of penetration experiments. I believe the intuitive “high velocity penetrates better” is true against many forms of cover, but sandbags and other highly dense (and abrasive?) cover do tend to disintegrate the highest velocity rounds, giving a slight edge to lower velocity bullets.
Fluids (which sand is to some extent) exhibit square law behaviour with regards to speed. Belly flop off the edge of a pool, no pain. Belly flop off a 3 meter platform…big problem.
A couple of other factors influence the behavior of the projectile leaving the barrel. Spin rate and center-of-gravity of the projectile influence the stability. Some bullets wobble significantly if not matched to spin rate applied by the barrel twist. After traveling some distance, aerodynamics may clean up the yaw a little bit.
On the 5.56mm front, the older round developed for the M16 (M193/M196) were lighter with a more rearward CG. The rifle also had a 1 in 12 twist. One revolution in 12 inches of travel in the barrel. This round fired in the current M16A2/A4/M4 rifle wobbles severely with very limited penetration. The later rifles have a 1 in 7 twist giving a higher revolution to the bullet. This reduced penetration is actually useful here in Italy where some ranges require it to keep from penetrating the existing range barricades.**