Conclusions

One big surprise when performing this second set of tests was how much they differed from the previous set of tests. All the bullets tested penetrated more walls than before. Softpoint rounds that were previously stopped in the second wall sailed through the third largely unmolested and even the 40-grain varmint rounds showed greater penetration than before.

Exactly why this change occurred had us flummoxed for the longest time because we were bad scientists, changing both the barrel and the drywall thickness at the same time. The last test used a Mini-14 with a 1:9 twist barrel shooting at 5/8-inch drywall. This test used an AR with a 1:7 twist barrel shooting at 1/2-inch drywall. Was it the thinner drywall that failed to break up the bullets as much? Or did the faster twist rate stabilize the projectiles to the point that they didn't tumble and come apart? Did the AR get greater muzzle velocity than the Mini despite having the same barrel length? Did all these factors contribute to some degree?

Now, several years after the tests, we have the answers. First, the twist rate of the barrel doesn't substantially affect terminal stability. This is discussed in this very comprehensive FAQ entry in the AR15.com ammo oracle. As for the drywall thickness, there was more in play than just the amount of drywall through which the bullet passed. A knowledgeable reader sent a informative email, reproduced in full here:

A big factor in your drywall tests was 1/2" vs 5/8". The extra 1/8" difference does make some difference, but the biggest difference is the density of the drywall. Almost all 1/2" drywall now made is "lightweight", while the 5/8 stuff was probably Type X. The type 5/8 is almost 30% more dense than the 1/2. The 5/8" type X also contains glass fiber reinforcement. I doubt the glass fiber changed much, but the increased density over the more porous 1/2" certainly did. That extra 1/8 plus the extra density means that each sheet of 5/8" drywall should have almost the same protection as 2 sheets of lightweight 1/2".

Moving on to other calibers, some results were surprising and some didn't come as much of a shock at all. Both types of 5.7x28mm ammo tumbled without fragmenting, perhaps due to lower muzzle velocity than .223. (5.7x28mm out of a 16-inch barrel should be only slightly above 2100 FPS.) Bullet construction may have had something to do with it.

The .308 168-grain A-Max bullets blew through all three walls like they weren't even there. Color us surprised. We're still interested in seeing how the 110-grainers do, though.

One thing that can confidently be said about this test's results is that even though .223 ammunition is affected by interior walls to a greater degree than other calibers, minor differences in wall thickness and firearm specifications produced notably greater penetration. Applying these conclusions to your own personal firearm is not a valid assumption. Even worse would be to assume safety for anything in the line of fire regardless of how many walls intervene.

Next Time

With respect to the testing parameters, the half-inch drywall provides better worst-case parameters for penetration, so we'll stick with that. It doesn't hurt that it's slightly lighter than 5/8-inch sheets, too.

As mentioned above, changing both the twist rate of the barrel and the thickness of the drywall makes it unclear which was responsible for shooting through more walls than before. We'll have to use both 1:9 and 1:7 twist barrels on the test walls next time. It might be worth trying out different barrel lengths, too. Will the additional velocity of a 20-inch barrel be enough to offset the increased stabilization imparted by the velocity and 1:7 twist? What about higher velocity but less stabilization (i.e. a 20-inch 1:12 barrel)? Varying twist and barrel length independently makes it possible to look at stability and velocity as separate factors.

Finally, there are a couple more cartridges that need testing. 5.45x39mm is another popular high-velocity round available in a number of bullet configurations. I also suspect that 110-grain .308 varmint bullets might do something interesting in drywall. There's only one way to find out for sure, though!