Sunday, March 4, 2018

Am I Reading This Correctly?

I have given some thought to how to protect students in active shooter situation.  My thought is that several layers of tables turned into barricades with a 70 degree angle to the entrance to the classroom would, by the fourth or fifth layer, have slowed down most handgun rounds (which are by far the most common in active shooter incidents) enough to deflect them.  What I found:
When reconstructing a bullet's trajectory prior to impact using the spatial orientation between two consecutive bullet defects (e.g., by probing), it is important to take the bullet's deflection into account. The (critical) ricochet angles as well as the vertical and horizontal deflection angles of eight cartridge types on laminated particle board have been studied. For all eight of the cartridge types combined, the critical ricochet angles lie between approximately 14° and 26°, while for the subgroup of the jacketed bullets, this range lies lower, between approximately 14° and 18°. The data from this study can be used to assess the accuracy and precision of the applied method. The results show that the highest deflection angles are seen near the critical ricochet angle. Generally speaking, vertical and horizontal deflection angles can almost be neglected above angles of incidence of 30° or 40° for handgun ammunition when shooting at laminated particle board.
Am I reading this correctly, that 70 degrees would maximize potential for deflection, once the bullet has expended enough energy on the first few tables?  Obviously, the instructor still needs to be ready to return fire.

Also what is the penetration depth of 9mm FMJ in plywood at distances of a few yards?  I have done experiments with sections of gypsum board on pine studs construction and even .380 ACP JHP would go through six layers of gypsum board and disappear into the ground behind it.  I would expect plywood would eventually stop a bullet, but how many layers?

This blog is full of interesting materials and bullet tests.  Curiously, 9 layers of gypsum board stopped a .357 Magnum JHP; frangible .223 made it through 8 boards.  My guess is that eight layers of 70 degree angled desks would stop even many FMJ pistol rounds.


Will said...

I think that info may be available on "Box of Truth" blog. Lots of material testing with ammo there.

John E. Jablonski said...

Seems like 223 is actually LESS likely to go thru a wall and hit someone.

BFR said...

1. When firearm projectiles are fired and ricochet in shallow angle environments (think of hallways, or conversely, the hood of a car), without performing any mathematical analysis, the overwhelming anecdotal information shows that the projectiles deflect from the struck surface but then continue on in the same original vector, traveling parallel to but a few inches above said impacted surface.

2. 5.56 mm (most Mil/LE use 5.56 not .223) is less penetrative in residential construction materials than typical handgun projectiles. The velocity of the carbine ammunition creates early yaw and fragmentation, thereby limiting penetration.
This fact is one of the data points that drove the change for LE close-quarter/SWAT entry from 9mm SMG's (MP5 et al) to the 5.56 carbines in the mid-90's.

Nathan said...

I was going to suggest Box-o-Truth" as well.

StormCchaser said...

Why not just get everyone out of sight of the door? Unless there is a ricochet hazard, then rounds through the door won't hurt anyone.

Clayton Cramer said...

The doors do not lock.

John Moore said...

Yes... but putting the tables in front can block entrance, even if they don't stop the projectiles. Unless the perp is going to expend a few magazines just trying to weaken them so he can push the door open.

Clayton Cramer said...

John: Unfortunately door opens out to hall so it will not block.