Hello Andrei,
One that I do think would be more lethal is the mounted Gatling M134 (that Terminator made famous), see http://en.wikipedia.org/wiki/Minigun. That fires up to 6000 rounds/minute which is pretty crazy. I think a salvo of that would have made a trench through the Orcs, the same bullet killing or maiming several of them.
IIRC the 7.62 NATO doesn't have that much penetration (more than the 5.56), enough to do in an Orc, but I don't think it would get the next in line, particularly if the Orc in question has armor on his back.
