The tires work well. Make certain that you compress them together so that there are NO air leaks between the tire walls. We just used some light cable and turnbuckles to compress them at 5 points equally distant around the outside of the tires, in a loop around the outside and back through the inside.
It is also a good idea to cover them with a tarp, heavy plastic, etc. The one I've used was about 20 feet long and did an excellent job.... almost NO sound except some almost staight out towards the target area and some behind the shooter. The end of the barrel should be as far inside the tire tunnel as possible.
The KEY is to treat the sound like water in a swimming pool. Even a small hole to the outside will leak a LOT more than you expect.
I will be building another one next year with my shooting buddy on his new farm. We will make a frame to hold the tires, form the inside with scrap plywood, several sheets thick, and just slightly larger than the tires. We will then blow cellulose insulation, used for homes, into the frame to surround the tires, under maximum pressure available from the home insulation machine.
Fortunately we have one of the best sound engineers in the country as consultant, who works with Nassa, DuPont, and all the car manufacturers - in the area of sound attenuation. He says... don't let any sound out of the structure, just like you would pay attention to holes in a swimming pool.
You WILL also gain a LOT of attenuation if you make the opening and exit holes smaller.
NOTE: you will not be able to see what is coming across the line of fire so PROTECT YOUR LINE OF SHOOTING with a secure fence, etc. One of the people using our old tire tunnel made that finding when a neighbor wandered into his yard to see what was making all the noise... it was the bullets slapping the target. He did not hear the gun shots until he was VERY close to the line of fire.
eek: