I can't for the life of me think of what they call the bullet traps we used on submarines when off going watches unloaded their weapons.
Essentially, it's a cylindrical pipe configuration with, I believe, a rubber insert on one end to place the barrel of the weapons while you're manipulating it.
What's inside, I don't know. I'm not having much luck finding this thing with a google search. Maybe someone else might have more insight on this.
At any rate, this shouldn't be too difficult to construct. A suitable pipe, maybe 4 to 6 inches in diameter, with a rubber grommet-style insert on one end in which the barrel may be placed. Inside, fill it with sand and, to keep the sand from becoming a "back splash" problem, put a solid layer of rubber over it, maybe a half inch to an inch thick.
The bullet will pass through the rubber, letting the rubber seal up behind it, and keep the sand from splashing back.
If you want additional sound isolation, you can build a boxed in enclosure over the mouth of it which will be lined with sound baffles. Make it big enough to stick your hand and pistol entirely inside through a hole in the enclosure.
Simple, inexpensive, easy to make and maintain. And you can make it very portable and easy to stow without taking up a lot of space.
