Terminal velocity
 

I'm watching this TV show and they're doing a test to see which round has better penetration: 5.56 or 7.62. They're firing at 16 acrylic plates packed tight together but they're only firing from about 10 yds. It got me wondering: at what range does the average rifle round reach it's terminal velocity? It seems to me that, at 10 yds, the round is still speeding up. Am I wrong? It the round going as fast as it's going to go as soon as it leaves the barrel?

Well
 

For the most part as soon as the projectile leaves the bbl it immediately starts slowing down. Air resistance(drag and of course gravity) affects the bullet immediately. In fact with some rounds the bullet could actually be slowing before it reaches the end of the bbl. It can happen with longer bbls with some handgun rounds and maybe 22's...especially with fast burning powders...Or so I've been told:D

The short answer is: the bullet's highest velocity will be right after in leaves the barrel.
In other words, as soon as the bullet exits the barrel, the bullet starts slowing down.

Once it leaves the barrel the only forces acting on a bullet are gravity and friction. It will slow down and start to fall.