Joel Skousen writes in his book “The Secure Home” that a gravel-filled wall is better than concrete, for an exterior wall or an interior safe room. While persistent impacts will drill a hole in concrete, they will have no effect on gravel, except for slight settling and spillage, generating a gap only at the very top where protection is not needed. Gravel (1/2 to 3⁄4 inch, presumably fragmented and not rounded pea gravel) will deflect and destroy most rounds, unlike sand, which merely slows most rounds. In his book “The Secure Home”, Skousen advises using 5/8-inch or 3⁄4- inch plywood sheets screwed to both sides of steel studs to contain the gravel. (Wood being essentially 2 inch gaps that are transparent to many types of rounds.) Skousen also speculates that a hollow heavy steel door could be filled with gravel. - Mr. Bravo
JWR Replies: "Skousen Walls" do work well, and I recommend them for anyone that wants to do a "Harder Homes and Gardens" upgrade to an existing wood frame house. A couple of years ago, I got a briefing and a slide show from a friend that did some actual shooting tests with up to 12 gauge slugs on dummied-up wall sections. (He expended over 400 rounds in the tests.) He proved that 3/4-inch plywood walls filled with "Three quarter minus" road rock gravel (rough crushed rock that has been screened to be 3/4-inch or smaller) works best for a Skousen Wall. And Mr. Skousen is correct that a wall filled with just small pea gravel or sand will drain like an hourglass after a number of large caliber rounds impact inside a 6" radius.
And as for ballistically protecting doors and windows, there is no substitute for mass. As mentioned in my novel, I recommend using five stacked thicknesses of 1/4-inch steel plates. (These thinner plates are much easier and safer to maneuver for construction than a single one inch thick plate.) Yes, we are talking about a lot of weight. (See my novel Patriots for a handy formula for determining the weight of plate steel.) Hinges must be sized accordingly, so plan on using vault door hinges. BTW, the hinge support for this kind of weight, requires either a 6 inch I-beam post with an anchor bolt footing or a fully reinforced masonry wall (with a grid work of re-bar) supplemented with a 1/4 inch plate that is at least 4 inches wide, running vertically.) If you aren't mechanically inclined and are willing to pay a bit more, you could of course also by a commercially made vault door.
Lastly, regardless of the door design that you choose, keep in mind that a "decorative" 20 inch thick masonry wall +/-6 feet forward of your front door is cheap insurance that your front door won't come under rifle fire from looters except up close and personal. (And then they'll probably be reluctant to subject themselves to ricochets.) BTW make sure that the wall is at least three times the width of your door. For those of you on a budget: Buy a lot of sandbags. They are sometimes available through military surplus stores, but the best way to buy them is to bid on a lot at a DRMO surplus auction. BTW, DRMO auctions are also a great place to pick up concertina wire at near scrap metal prices.