Mr. Rawles,
On the subject of securing windows with plywood. there is one piece that seems to be missing as may often be the case of a “My Home is My Castle” scenario. What if you have to get out? While there is no 100 percent answer, there is the military and historically proven fact that if you make a defensive area/building too hard to get into, it is too difficult to get out of. Such an example would be if there is a fire in the structure. Thanks for the Blog and your great work. – Grog
JWR Replies: Window shutters should be designed to be latched from the inside, to allow emergency egress!
Dear Jim:
For ballistic protection of windows the best solution is to use ballistic steel – a special high hardness and heat treated formulation.
Ballistic steel plate is only 1/4″ thick (6.4 mm) and tested to stop six rounds of 7.62mm NATO full metal jacket (FMJ) [“ball”] per square foot. It also easily stops 7.62x39mm AK-47 mild steel core.
At roughly 10 lbs. per square foot, your 36″ by 36″ piece would weigh only 90 lbs. versus the estimated 360 pounds for 1″ of regular steel!
Flat ballistic plate in quantity is much less expensive than curved steel rifle plates.
But of course would be more expensive than regular, mild steel. But this extra cost would be offset by reduced installation costs, as you don’t have to build to handle four times the weight.
More importantly, a 90 pound shutter could be closed and secured much more quickly in a threatening situation. And closing a lighter shutter is a job that kids, or a petite woman could do – perhaps not possible with a 360 pound beast.
We have supplied customers in multiple applications, armoring doors, windows, safe room walls, etc., etc. High hardness steel plate is not something you want to be drilling or cutting – but can be ordered pre-cut, and pre-drilled.
Yours truly, Nick
Manager, BulletProofME.com Body Armor
JWR Replies: The problem with using a single 1/4″ plate is that although it will stop soft nose and standard lead core “ball” ammunition, it won’t reliably stop 7.62mm NATO or .30-06 armor piercing (AP) bullets.
The following data was found with web searches. Notably, the most reliable information was found at the excellent The Highroad (THR) forums.
The following is data for various 7.62mm NATO AP loads:
“Hiternberger AP: FMJ, tungsten core, 9.7 g; V10 850 m/s; penetrates 10 mm armour plate at 300 m
Fabrique Nationale (FN) AP P80: FMJ; 9.75 g; V25 823 m/s; can penetrate 6 mm steel plate at 100 m
Chartered Ammunition Industries, Singapore Technologies (CAI) AP M61: V24 838 m/s; conforms to US Mil-Spec MIL-C-60617; penetrates 6 mm chrome nickel plate (Brinell 450) at 100 m
Adcom AP [essentially the same specs as U.S. Lake City arsenal] AP FMJ; [sintered] steel core; 9.5 g; penetrates 3.5 mm SAE 1010 steel plate at 100 m or 8 mm SAE 1010 steel plate at 450 m; V24 838 m/s”
According to U.S. Army manuals, .30-06 AP can penetrate .42″ (10.67mm) of hardened steel at 100 yards.
Availability: While 7.62mm NATO AP is fairly scarce, there is lot of .30-06 AP in circulation .You often see it for sale at gun shows. Back in the 1960s and 1970s it was priced about the same as ball.
Ditto for 7.92x57mm (commonly called “8mm Mauser” in the US) AP. Those AP bullets reportedly can penetrate 7 to 8 millimeters of hardened steel armor plate at 100 meters. Roughly one third of the military surplus 8mm ammo in circulation is either AP or AP Tracer (APT).
One other consideration is that “stopping” a bullet doesn’t necessarily make the occupants of a building completely safe. There is still the risk of spalling–where pieces of the the back side of the armor plate itself can fly off, forming dangerous fragments. So if you have less than 1″ of armor plate, I recommend adding a spall liner–typically this is a Kevlar blanket.
And then there is the penetrating capability of .50 BMG Ball, AP, APT, APIT, and Raufoss. Yes, there are a few rifles out there, but if you have .50 BMG bullets whistling toward your retreat, then they would probably be coming from a large, well-organized attacking force, so you’ better plan on exfiltrating, immediately.
Conclusion:
Just one hardened 1/4 inch hardened plate is insufficient foreseeable threats. A pair of hardened plates would probably suffice, but I’d feel safer with three or four. Multiple stacked plates work better than a single plate, because you benefit from multiple layers of surface hardening, rather than just one.
The other option is setting up 45-degree angled plates. This offers better protection with less thickness, but of course require more surface area and hence more weight. It also greatly complicates the mounting, to prevent someone angling a shot behind the plate. Thus, angled plates would be well-suited to buildings with extra-thick walls, such as adobe houses or Earthships.