Years back, I did some research on tunnel designs and problems. The subject is a lot more complex than meets the eye. There are a number of tunnels that are owned by companies clubs, and individuals. Rushing in, without the proper engineering is likely to result in the creation of a very expensive "monument" that does not work as well as it could have, and cannot be easily fixed. This is particularly true if you plan on using one to test rifles that are shot off of sand bags, that have to be re-aimed for each shot. Mirage is the issue. Any time that you have a surface that is warmer than the air next to it, mirage can and will happen, and it will move the target image, so that you will be shooting at different target locations from shot to shot. Fellows who lack experience with the sort of high magnification scopes that are required for competitive benchrest shooting will not properly understand this issue, so it will be easy for them to overlook it in the design stage. I have a friend who has a state of the art tunnel that has been used to test bullet designs. He is in the construction business, and it was still very expensive to build. It is underground, made of concrete (with a smooth tube interior, and features a large expansion chamber in front of the tunnel, as well as proper control of ventilation so that the target image is stable. It has been a while since I discussed it with him. I think that I will give him a call and ask him what he thinks it would cost to reproduce today. If I manage to get that done, I will post the information on this thread.
Added a little later: We are talking a half million to construct, a thousand a month in energy costs to keep the air conditioned to the proper temperature, and a lot of vacuuming to keep the unburned powder cleaned up. Unless you are using one a lot he said that it would not be worth it. You might look into what Bart Sauter of Bart's bullets has. I believe that he dug a trench, formed and poured the sides, and shoots out of his basement. My friend told me that if he were to shut down his conditioning system that it would take a month to bring the tunnel back to a stable temperature condition needed for critical work.