Oh okay
That particular rifle is the cat's ass. It feels
pretty fairly light and very well balanced, despite having an HBAR, A2 stock, and an ACOG on top of it. The A2 stock is a pretty good counterweight to the heavy barrel. Since that one doesn't have an extra mount, BUIS, or taclight it "feels" lighter and swings faster than either of the two pimped out carbines pictured above it.
I'm no expert, but to me a carbine is too heavy (or too poorly balanced) when I can't hold it comfortably with one hand while opening a door, or using my support hand for anything else besides holding the weapon.
I realize that I'm about to contradict myself a bit, but as far as utility goes balance is just as important as net weight. The biggest problem with the two carbines pictured above is that they are nose heavy from the Surefires in front and lightweight sliders in back. The ACOG doesn't really add to the problem though, because it's perched pretty much right over the ideal center of gravity.
Now that I've managed to confuse myself I'll sit back for a minute and see if my ramblings helped to answer your question