Wow. Finally, my experience as the Quality Dept Manager at USG can be of some good. I can give you some insights on sheetrock and the different boards and wall systems. First, modern 2x4 are 1 1/2 x 3 1/2, and that is pretty consistent with computers figuring out how to optimize lumber usage, and ultra thin blades doing the cutting. Modern studs are not as dense as studs were years ago. Reason? Faster growth rate of modern forests through management techniques, trees reach harvesting height faster than ever.
The sheetrock itself
is not solid rock. Voids are intentionally designed in to reduce the weight of the boards, and other materials are added in production. The size of the voids are pretty small and uniform in USG board, tending to be less uniform in most other brands. (Bigger, less uniform voids = weaker board, easier to penetrate). The paper-wrapped edged of the board are denser than the center - less air is mixed in to harden those edges to help prevent handling damage, but while they are harder, the edges are also more brittle to face strikes. The relatively large density differences between the edge and field mix does result in a weak area where they meet, approx 1 1/2 from the edge . The edged are also tapered to allow easy filling with joint compound.
If you have a fire rated board, that board has chopped fiberglass added to it, and the fibers run (mostly) in the length direction, the same as the fibers in the face and back papers. Strength of any wallboard product is
significantly higher across the width of the board as opposed to the length, but fiber glass increases the core strength even more.
Why bring all this up? How your board is hung can effect how a bullet might react to striking it. If your board is hung "up and down", a bullet hitting at an angle along the length of the board will meet with more resistance than if your board is hung length-wise with butt joints (where the flat ends meet), and the thinner, brittle edges of the board are directly in front of wood or metal studs. Board run length-wise has a seam about 4' from the ceiling (and another 4' from there if your ceiling is over 8' high) that is both weaker and thinner than the rest of the board. Board hung in this direction should also offer less resistance to bullet strikes along the length. Complicate the seam issue with the tape used. Paper tape = less strength than fiberglass tape.
We could also get into the reaction of the sheetrock core to a bullet strike. CaSO4 releases water in reaction to heat. It's the reason why it's fire resistant. In a fire, the major strength of the board is burned off quickly - the paper. Fiberglass does not directly add to fire resistance, but acts to hold the core together longer by spanning the cracks occurring as water is released in reaction to heat. A Firecode C board has vermiculite in it, in addition to the fiber glass found in Firecode board. Vermiculite pops like popcorn, releasing additional water vapor and expanding to fill in cracks.
Two hour rated walls have a double layer of board on both sides. 1 hour ratings have one. So depending on your local codes, you wall thicknesses may vary.
I know of no study looking at bullets striking sheetrock, but I'm trying to give a reasonable explanation based on what I learned from 15 years in the industry. Maybe someone will test some of this out...
Quick edit: I can see better potential for 5.56 fired at an angle to be deflected off the back of the board in the next room and into a stud based on horizontal construction for the above reasons. Straight on, all bets are based on resistance / tumbling and shattering. A bullet tumbling and hitting the next piece of sheetrock across the width will meet with more resistance than one striking length wise.
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