Considering almost every indoor range (civilian and military) I’ve ever used has had some variation on the “boiler-plate and sand pit” backstop, I think we can call it proven tech.
Obbn – I would check your local sportsmens league or possibly even the NRA for specs and plans.
I want to know where you are getting .22LR ammo. Seems to be a bit of a shortage lately. Scarce as Hen Teeth.
It’s been 23 months since I’ve seen .22LR ammo on store shelves, other than the occasional 50-round box priced at $8 to $12. I keep reading on the interweb that 22 ammo is/will become more available, but I have yet to see any evidence.
No kiddin’! I used to wander into WalMart and be able to grab as many 550 round boxes as I wanted for about $22 each.
Over the last year, I’ve been damn lucky to be allowed to buy one box, because I checked a store I happened to be passing by at 8:30am. My wife was waiting in the car, so I sent her in right after me, and she got damn near the last box! It was Remington, which I don’t prefer, but felt lucky just to get some.
At least they didn’t jack the price. Still about $25 after tax.
1/4 inch steel should be plenty for Hen Teeth. Unless they’re magnum, I mean.
My uncle built a cube of old railroad ties. 8’ wide, 8’ high, 8’ deep. It would stop anything, including an out-of-control pickup truck. :eek:
Overlapping, angled plates of steel over a sand trap is very effective and the lead and copper can easily be recovered by sifting the sand. Bang, clang, bang, clang. I win. You sift the trap. 
You can buy them. I have one that I used in my basement for air guns. .22 rimfires vaporize at that close of a range. Lead dust is nasty stuff so I didn’t use it much. The sticker that is on the bullet trap I linked gives a minimum distance, 70 ft. if I recall correctly.
At my new place I built a back stop out of logs with dirt piled around and behind.
And now I have a use for those old railroad ties that no one will take.
Thanks Straight Dope!
A mound of dirt is O.K. as long as the temperature stays above 32 F; below that, bullets will ricochet.
IMO, railroad ties are the best all around solution. That’s what I use. A pile of them will last many years, and will withstand any caliber.
You need a Grassy Knoll.
<hijack> what’s so dangerous about ricochets from a .22? From what I’ve seen on Mythbusters, bullets tend to spatter when they hit a solid steel plate, even at fairly shallow angles.