While this is certainly interesting, I believe there is a flaw in their testing method. I have never seen a house that had two walls spaced an inch or two apart that only had an inch or two of clear space between the drywall sheets.
For a projectile to travel through two walls of a normal house, it has to penetrate two drywall sheets spaced roughly four inches apart, then travel upwards of eight feet before penetrating another two drywall sheets spaced four inches apart.
And that's all assuming the projectile is hitting each wall completely perpendicular to the wall face. Even a slight angle of incidence to the face of the wall will create some seriously awkward ballistics.
A projectile loses energy every foot that it travels. It loses a significant amount of energy when it has to penetrated a medium that is 5/8" thick, regardless of its composition.
I have a feeling that most rounds, after traveling through a single typical wall will have a tendency to tumble once they come out the other side.
This test doesn't allow any of the projectiles to replicate any type of tumbling between walls. The projectiles immediately engage another sheet every inch or two, and therefore have no space for any tumbling effect to take place.
I would certainly be curious to see this same set of tests performed with a more accurate depiction of typical home construction with appropriate air gaps between each wall.
