I live in a rural area, but I have a neighbor about a mile away, across a river, about 30 degress off from my shooting range. There is no risk of errant bullets going his way, but it would be nice to cut down on the noise of some of the testing I do. I normally shoot from inside my shop out a garage door. How feasible would it be to make some kind of suppressor tunnel that I could shoot through and thus reduce the noise outside? I shoot with both earplugs and muffs while inside so I am not trying to reduce the noise to myself. It would not be mounted on a gun. I envision putting the muzzle of the gun inside a tube that is aimed toward my target backstop. What kinds of sound reducing materials would be effective in such a setup? Is such a thing legal?

This isn't a suppression rather a direction question. You could get a culvert and shoot through it

I've never seen it but heard of shooting through a horizontal stack of tires.

The stack of tires is a good idea as is two 55 gallon drums welded end to end with two 30 gallon drums inside with foam between them. Use a small shooting port in back to put the barrel in.

The BATFE says that this arrangement is not legally a silencer. I have a letter saying this.

Ranb

Years ago, when I lived much closer to town, I made a "rectangular tube" from a single sheet of 3/4" plywood, open on both ends, and on the inside hung pieces of carpet that had been cut like an upside down "u" from the top piece of plywood leaving a slot about 4" wide to shoot thru. This worked very well, but was a bit impractical as it required moving to and from my shooting bench. Also, if the ends were left open it collected so much dust that the first few shots created a huge dust cloud so it was a short lived project.

Karl

A few lessons based on other peoples experiences; make sure there are drain holes in the bottom, wash it out occasionally because powder residue can catch fire and check for wasp nests before shooting

this thread is a bit old now, but I found a good article on building "muffles" for rifle ranges by the military

military muffler testing

That approach is easily duplicated. I'm going to give it a try.

a Culvert just focuses the sound in a given direction

My vote would be for the horizontal stack of tires

Is it sufficiently obvious that stack of more smaller diameter (13") tires like compact car tires would be more effective than a smaller number of large diameter truck tires?

Many rifle ranges near populated areas make shooters shoot through a length of concrete culvert... it narrows the possible deviation from the desired path downrange.