The concussion rifle is a long, pointy-barreled contraption that can hold eight shots at a time (which, incidentally, gives it more total shots than any other heavy weapon). What it does is shoot out a perfect-accuracy shockwave that instantly travels to the target location and causes nasty lag.
When it hits the location (aim at the ground, not the enemies themselves), it ruptures the ground and kills everyone in the surrounding area. The reason I call this a one-man siege weapon is because the effect works through walls. Just aim at the wall right below the feet of those pesky wall guards and let 'er rip. They'll fall off by the droves.
The tricky part about using it is that aiming a "perfect accuracy" gun is stupidly hard. This is because the shockwave will travel straight out of the gun's barrel and completely ignore the aiming reticle. What you have to do is zoom in to first-person and then try to aim it based on the gun's graphics.
Note that this thing can be shot regularly using the aiming reticle. This is because it actually shoots two projectiles. One is the AoE shockwave that kills everything it hits, and the other is an invisible 100 blunt damage projectile which is the actual shot. The shockwave is just an effect that happens whenever you fire the gun (modding oddity).
Just note that the concussion rifle really doesn't have that great of a range, and that it will cause major lag if you attempt to fire it great distances. The rocket launcher technically has the longest range of the heavy weapons, you just need to deal with the firing arc.
Once you get the hang of the concussion rifle, you'll be able to take on entire armies by yourself, and cripple a planet's defenses before your troops can make it up to the fortifications. One nasty little machine right there...