I have a pretty good friend that has an indoor range in his home. He uses it nearly every day that he's home, and can get in some time down there.
The shooting station is 4' wide, 7' high & 30 yards deep. He has it marked out every 5 yards. It has lighting all the way down so it's bright and easy to acquire the targets at any distance thru the tunnel. The lights are built in a steel box to protect them too. There's a shooting bench that folds down and is strong enough to hold his shooting rest. He has a high precision Pellet rifle that he competes with, and the 25 yd range is perfect for Benchrest shooting practice. He mounted a spotting scope on the right side wall, so it can be used for sighting the shots all the way down the range.
The range is completely soundproofed, so when you're not in the shooting room you can't hear anything. There is an intercom for an emergency.
Also, he never uses it when he's the only person in the house. He tells his wife whenever he goes there, unless he's with somebody else down there, so there's a level of safety that goes with using it.
It's basically a 12" thick concrete tunnel with fiberglass reinforced concrete.
The ceiling is plated in 1/2" steel for the occasional stray bullet.
The floors and walls are covered in 1/2" crushed rubber pads to keep down any sound.
He had a steel bullet trap built into the far end. The trap alone cost him nearly $20,000. It is a Venetian Blind trap, like the one in the link above showing the different trap designs. It'll stop any handgun round up to a .44 magnum round, lead or jacketed, and will not ricochet. He estimates the total cost for the whole setup was close to $30,000.
I've been shooting with him numerous times and it's a sheer pleasure to shoot there. Just behind the shooting station is his entire reloading setup. He sometimes loads his pistol ammo for testing as he shoots.
He has quite the setup.
It has a reverse draft exaust so the smoke is drawn down to the far end, and exausted out thru a pipe ending at the roof line.
Here's a picture of me with my tommy gun, shooting at 25 yds. Since it shoots .45 ACP ammo, it was safe to use down there.
My next home will have a full 200 yd. outdoor range with 5 shooting stations.
There's no way I'm up to setting up an indoor range. It's way too expensive to do it right. There's much more to it than just setting up some railroad ties as barriers and putting in an exaust fan to clear the smoke.
I wish you well in your endeavor.