Try lining the first 10 feet of the tunnels with Sonex acoustical panels to moderate the reverberations. Use the outdoor/paintable stuff so it can be hosed off monthly to remove powder residue.
They should have used the 6- or 7-foot tall rectangular concrete tubes. Plus, the surface facing the shooters should be fully lined with 2x6s on the flat to absorb stray rounds and contain the bullet/impact splatter.
I understand the idea and have wrestled with the execution myself. In the interim, get some platforms made for kneeling/sitting & prone practice. Maybe even dig a 3x3 foot pit to the appropriate depth (with a hinged cover so it's never lost or improperly replaced!) for standing and pistol shooting. These things have been done before for basement culvert ranges.
[This message has been edited by Cheapo (edited August 18, 1999).]