I saw that too. I don't think they said who it was but who else would it be?? I heard he has a range in all his homes. I know there was another thread that was about the article and stuff but can't seem to find it.
The entire lane doesnt need to be 8' ceilings. Use precast concrete tubes, like for storm drains, large enough diameter to run a cable system for your target(s).
The "tube lanes" could be installed under any normal concrete slab, or even buried under your yard. Bullet strikes would not be a problem. Your trap system could also be greatly simplified.
The construction of your "behind the line" area doesnt need to be any more involved than a simple concrete pit, like for changing your oil. Covered to reduce sound of course.
Something like this could be installed in almost any basement, with a large enough yard. I'd recommend building a containment room to shoot from to reduce noise, to ventilate, and protect other residents from NDs.
If i were pulling permits for your "Underground Archery Range", i wouldn't care what you used it for after i was paid and left. And i would make sure all your neighbors stayed away from the backyard "septic leak" that i was "repairing".
You're not building it under the garage, you're building it under the workshop at the back of the garage. This simplifies the structural load of the slab for the cars.
Soundproof and ventilate. Don't skimp on either. And make sure you have adequate range lighting.
And you might want to consider a match-grade air gun. It solves a lot of problems with noise and ventilation very neatly.