Recently I watched Full Metal Jacket again, and during its city attack scene I noticed something that I've been wondering about for quite some time now: how are the effects of bullet impacts in concrete walls created?
For example, in the scene I'm talking about (linked here) Adam Baldwin shoots at a building, and you can clearly see it gets ripped to shreds by bullets. In this scene, I can imagine that live bullets are used, because you can leave the building empty and actually shoot at it.
However, I just watched Equilibrium the other night, and in this movie a lot of gunfire takes place inside buildings with actors all over the place (linked here). Here, use of live ammo is ofcourse impossible.
I've been searching around for an answer, but most threads are about bullet impacts on actors (blood with squibs) or through wooden walls (also squibs). However, none talk about concrete walls. Are the holes pre-made, the squibs inserted and then covered up? If that's the case, are they remotely detonated?
Some effects seem to also involve high pressure, but I can't imagine that drilling holes and rigging every single bullet hole to a high pressure machine is very feasible (especially in the case of Equilibrium, where dozens of bullet holes are created in a matter of seconds).