Finish Out of PRB’s 100 Yard Underground Range

Hey, guys. A ton of readers and friends have asked for an update on my underground range. A couple of years ago, I published an article that shared all the details of a 100-yard rifle range that I was building at my new house. I was already under construction, but I also asked my readers for input on how to finish it out. And wow, you guys were helpful! I actually changed quite a bit based on comments from that post. Thanks to all of you who weighed in!

The initial construction was complete, and the tunnel first went into use in December 2022, although I’ve still been working to instrument and outfit it. I like to tinker, so I’ll continue to improve it forever, but I’ll go ahead and share a walk-through of the tunnel as it sits today and briefly share what I’m considering adding in the future.

The original article shared why I was building this, drawings, construction photos, and more. I won’t repeat that content here. I might suggest starting with that article or at least referencing it if you are wondering about something that wasn’t covered. Original Article: https://precisionrifleblog.com/2021/12/27/100-yard-underground-range/

Here is a quick walk-through video:

Now, I’ll walk through a few specifics that I changed since the plans I shared in the original article.

Professional Ventilation Design

There were a lot of comments about the air ventilation in my original post. I felt like I’d read a lot about range ventilation design and had already talked to several experts – but it became apparent that it was a very specialized field. Lead poisoning is a serious health concern for indoor ranges, so I decided to hire Carey’s Small Arms Range Ventilation to fully engineer/design a range ventilation solution for this setup. Carey’s has completed over 1,000 indoor range ventilation systems! They provided complete drawings, specified all of the necessary equipment, and coordinated with my local HVAC installer. They also guaranteed the system would meet all current EPA, OSHA, and NIOSH standards for air quality.

The first design they came up with was extremely complex and would have required that I drop the ceiling height in the shooter’s room from 9 feet down to 7.5’ – which would have dramatically changed the feel of the space. It also required $25,000+ worth of equipment! The airflow that this system was capable of moving was drastically over-engineered for my project, but here are the plans for that original design they provided, in case it’s helpful for others who might be considering a larger space.

I asked the engineer I was working with if he could discuss this project with other engineers to see if there were alternatives. Through some collaboration, the Carey’s team was able to dramatically simplify the design and still accomplish the airflow requirements we were targeting. So here are the plans that I ended up implementing in my underground range: Final HVAC Design & Equipment from Carey’s Small Arms Range Ventilation. 

Here are a few keys I learned about range ventilation:

• I initially thought too much airflow couldn’t be a bad thing, right? But, too much airflow often creates air turbulence and backflow of contaminated air – meaning harmful air could end up back by the shooter. What you actually want is “laminar airflow,” which means a smooth and continuous airflow that is very uniform in one direction. That is how ventilation systems are designed to work in labs and surgery rooms. For an indoor range, you want the air to flow from behind the shooter all the way down to the target and then exhaust out – all in a very steady, uniform way.
• To get that steady airflow, you always want to pull air from the target – and not push air from the shooter’s position.

I’m happy with how the ventilation system works today. It turned out that I didn’t need to move near as much air as I had originally planned. There was so little air that we were able to do away with the fresh air intake, and the ventilation system simply pulled conditioned air from my shop that is directly above it. The HVAC units we used to heat/cool my shop were big enough that they’re able to keep up even when the range exhaust fan is on. That was a huge simplification for this whole project.

The exhaust fan is installed directly above the target (shown above) and is perfectly aligned with the tunnel so it doesn’t create turbulent air. The speed of the exhaust fan is adjustable by turning a dial on the wall inside the shooter’s room. I almost always shoot through the 12” x 12” porthole with the blast doors closed, which means I really only need to move around 1 mph of air through that small 1 square foot area and can run the fan at a low speed to achieve that.

There is at least 3 feet of dirt above the roof of the tunnel, and I insulated the shooter and target room – so the temperature stays surprisingly stable in them year-round. However, I can open the blast door to the shooter room and turn the exhaust fan speed all the way up if I ever need to flush out the tunnel to make sure the air temperature is perfectly uniform throughout.

Measuring Velocities: The EVO Ballistix EB-MK ONE

The biggest delay in posting this update on my tunnel was related to finding a reliable and accurate way to record velocities. I was initially hoping that my Lab Radar would be able to record velocities in the tunnel. I was fortunate to get connected with Guy Desbiens, the president of Infinition (the company that makes the Lab Radar and also makes high-end military-grade radars), and he thought it might be a challenge to get the Lab Radar to get readings in my range. He did give me a few tips for how to configure and position the Lab Radar to give it the best shot, and he was genuinely curious to see if it worked. I tested it in the tunnel before we enclosed the basement, and it did give me readings – but once the space was finished out, I tried it again. After trying it in several positions, I found a way I could occasionally get readings, but they were off by 1000 fps. The signal-to-noise ratio in the radar data was simply too high to produce accurate velocities.

I also tried the new Garmin Xero C1 Pro Chronograph in my tunnel, and it also wasn’t able to provide any velocity measurements. I would suspect that the radars may not be able to sort out the signal from the noise, which may be related to the enclosed space and/or the corrugated pipe.

So, the only way I could record bullet velocities in my tunnel for a few months was if I used a MagnetoSpeed v3 Chronograph. I really didn’t like that as my solution because you have to hang that device from the barrel – which impacts barrel harmonics and changes your point of impact. So when you use that device, you can’t shoot groups or check your zero and record velocities at the same time.

My goal is to find a very accurate and reliable way to measure velocities without having to attach anything to the rifle. Guy said that Infinition makes a 35GHz radar, the BR-3503, that would for sure work – but it’s around the $50,000 price point. That’s quite the jump from the consumer-grade devices I’ve been using, so I wanted to explore other options before I went in that direction.

After I published that original article, Marc Erneke from Germany reached out to me and said they were in the early stages of developing a new Doppler radar. I had a Zoom call with them, and they showed me the product and talked through the capabilities. At the time, I was still hoping my Lab Radar might be able to get accurate readings, but once I realized it wouldn’t, I reached back out to them.

Marc was supremely confident that their radar would get accurate readings in my tunnel. In fact, he told me they’d successfully measured velocities in tunnels as narrow as 2 feet in diameter! Their radar doesn’t function like the Lab Radar or other consumer-grade Doppler radars. The use of the constant wave Doppler radar technology allows for dramatically more measurements of the returning radar signal. It also takes most of its measurements within 30 meters of the rifle, meaning there would theoretically be less interference from the tunnel.

So, I ended up purchasing an EVO Ballistix EB-MK ONE Doppler radar, which is made in Germany. This radar is definitely a professional-grade device, and I paid $17,200 for the unit. The EB-MK ONE radar is currently being used by the German military, HK, CZ, and others. It took a few months for me to actually receive the unit since it had to ship from Germany and be imported through US Customs. Once I received it, Marc helped me get it all up and running, and sure enough, we were able to get accurate measurements!

I haven’t used the EB-MK ONE much at this point, so I don’t feel like I can give an honest review of it yet. It does so much more than simply measure velocity. It has a TON of advanced analytics capabilities, including access to the raw radar data, spectral analysis of shot exit events, the ability to build custom drag models, and much more. They even say that because the unit uses constant wave Doppler radar technology and has a high measuring rate, it is even possible to detect even smaller particles along with the bullet, like tiny kernels of unburnt powder leaving the muzzle. The unit takes up to 20 million measurements per second, which means the EB-MK ONE is capable of detecting the slightest changes in the signal, which allows you to analyze the spin and tumble (pitch/yaw) of a bullet.

I hope to get to spend some time exploring the EB-MK ONE advanced capabilities at some point and may do a full review in the future.

Bullet Trap & Target Room

My original plan for a bullet trap was to build a wall of ballistic rubber blocks, like Range Systems Dura-Bloc. I researched lots of options, and in one of those conversations, I got some helpful advice from an industry pro that as those ballistic rubber blocks absorb bullets, they will bloat. After absorbing thousands of rounds, they will become so rounded that the wall would no longer be stable. That makes sense when you think about it. Those are dense rubber blocks to begin with, so those bullets have to increase the size of the block over time.

I still liked the idea of a bullet trap that completely encapsulates the bullet. Traditional indoor bullet traps are usually often just a big pile of pieces of loose rubber. When a bullet hits that, it tumbles and comes apart, exposing the lead core – and lead exposure is a very serious health concern. When an indoor range has to do maintenance on a traditional bullet trap, they typically have to hire a specialty service to do it with Hazmat suits on. I wanted to be able to go downrange to do maintenance or check the target without a high risk of lead exposure.

What I landed on is The Bullet Bunker Goliath 4’ x 4’ bullet trap, which is rated from .22 rimfire up to 375 RUM. I verified with the manufacturer that it could take a 300 Norma Mag and 338 Lapua Mag because those are the largest cartridges I plan to use at this point. I do own a 375 CheyTac and 50 BMG, but I’m pretty sure I’d rather shoot those outdoors than in an enclosed space.

The Bullet Bunker design is kind of a mix between the ballistic rubber blocks and a traditional bullet trap with loose rubber. The unit still fully encapsulates a bullet (meaning no lead exposure in my target room), but inside the box are small pieces of rubber. The bullet trap can take over 10,000 rounds before any service is needed – and potentially a lot more, depending on the calibers and how concentrated the shots are. The hinged collar design makes the maintenance very straightforward. I bought a replacement panel for $300, which might not be needed for 10,000-50,000, and they threw in two 50-pound boxes of ballistic media, which they said should be all I need for at least 50,000 rounds. That sounds like a lifetime!

I bought the base and casters for the Goliath bullet trap, which perfectly centered the bullet trap in my 8-foot diameter tunnel. The green deflector around the outside is 48” x 48”, with the shootable rubber area measuring 42” x 42”. The fully assembled unit weighs over 2,000 lbs. (View drawing with dimensions)

Since the tunnel diameter is larger than the bullet trap, I continued with my original plan to add a 1/2” thick AR500 steel plate on the wall behind it – just in case a bullet ever misses the 4’x4’ green square. I honestly can’t remember a time I was firing at a paper target at 100 yards and missed it by feet – but just in case. On top of the steel plate, I added 20 Range System Dura-Panels. Those are 24” x 24” panels of ballistic rubber that are 2” thick, and I glued them onto the plate with the 3M 550 adhesive that Range Systems recommends. Those panels can absorb and encapsulate bullets and fragments, but they also reduce the echo/sound reflection dramatically.

I did get an extra large 36” x 96” roof hatch in the target room in case I ever want to replace the bullet trap. In fact, we used a crane to lower the Goliath bullet trap through the roof hatch to set it in place, which worked well. The roof hatch serves as a 2nd exit to the tunnel since the tunnel also serves as a tornado shelter for my family and up to 500 friends. 😉 I keep an extension ladder in the target room for such an occasion.

I wanted very flat/even lighting on the target, so I used LED light strips and mounted those around the mouth of the tunnel. They are all connected to specific plugs that can be switched on/off from the shooter’s room. The lights along the inside of the tunnel are also on a dedicated switch in the shooter’s room.

For now, I’ve simply attached a corrugated plastic sheet and paper target directly onto the bullet trap. My plan is to build a free-standing steel target stand in front of the bullet trap, but I haven’t quite decided how the target system will work. I’d like an electronic target system that will display the shots on a screen next to the shooter, and also run analytics on the groups (like average distance to center) and save the target digitally along with meta-data like date, rifle, ammo, etc..

I’ve researched several systems, and it appears that the Kongsberg Target System is the gold standard. It’s used by the US Army Marksmanship Unit, Lapua, and in several of the most prestigious competitions in the world. However, it is not without its flaws. It doesn’t appear to do everything I’m hoping for, and I’ve heard that Lapua isn’t really happy with it. I want the target stand to be able to seamlessly integrate and accommodate whatever electronic target system I end up going with, so I’m just using paper and walking downrange until I settle on the system.

The biggest concern is finding something that works indoors and allows multiple aiming points. If you fire thousands of shots in the very center of a target, you will destroy the center of the bullet trap. Also, since I’m shooting precision rifles into a 42” x 42” shooting area at 100 yards, it only makes sense to use multiple aiming points, similar to all the little aiming dots I have on the target now. Finding a solution that can do that and function in an indoor range may be difficult.

If nothing else, I actually have a degree in Computer Science, and I could use a remote camera to capture the images and write my own software to do the analysis and save the output. I’m hoping not to resort to that. Honestly, using paper targets just isn’t that inconvenient for me at the moment, either.

If you guys know of a good system that might meet my needs, please let me know in the comments!

Blast Doors & Acoustic Panels

I personally designed the blast doors, which are the two doors between the shooter and the tunnel. The idea is that you stick the muzzle through a porthole, and the bulk of the concussion happens in the “expansion chamber,” which is what I call the room between the shooter and tunnel. This is a design that a friend at Accuracy International suggested, and it works amazingly! I think it was a game changer in terms of reducing the concussion the shooter experiences.

The blast doors are made from 80/20 parts, which is like a professional-grade erector set that many labs use for custom fixtures, equipment, custom benches, or other uses. It’s based around T-slot profiles made from aluminum, but they seem to make anything you could dream up! They have all kinds of accessories, like mounting brackets, hinges, the ¼” clear polycarbonate panels I added, door handles, gaskets, and all kinds of stuff. It’s a rabbit hole for an engineer!

What I like about the 80/20 is that it is not only modular but also extremely extensible. I can basically directly mount anything to these doors. If I ever want to directly mount a radar, display monitor, slow motion camera, or other equipment, I could attach those virtually anywhere on these doors – and in a variety of ways.

This was another part I hadn’t ever seen done, so it felt risky – but the doors worked great and were everything I was hoping they’d be. I worked with Shepherd Controls (a partner with 80/20) to figure out exactly what parts and quantities I needed to pull off my design, and then I assembled all of it. Here is an itemized order with all the parts for my blast doors.

I hung two acoustic blankets on each side of the blast chamber room. I sized the room so these 4’ wide x 8’ tall blankets hang freely without touching the floors or walls, but they only had a couple of inches of clearance. I bought AlphaSorb Dual-Sided Absorptive Sound Blanket in 4’x8’ panels from Acoustical Solutions, which is constructed using two layers of 1 in. thick specialized acoustical fiberglass batting and features a durable vinyl-coated fiberglass cloth facing. These blankets have an NRC rating of 0.85, which means they’ve been tested in a lab and shown to absorb 85% of the sound over all frequencies the human ear can hear! If you compare that to other products you typically see in indoor ranges, you’ll see that is ridiculous performance. The vinyl coating is especially nice so that you can easily wipe them down as needed.

I was originally planning to add acoustic panels to the ceiling above the shooter and in the blast chamber, and I also thought I would likely add some on the walls on either side of the shooter. But, after shooting in the space for a while (even with most of the rifles being outfitted with a muzzle brake), I think the blast chamber and acoustic blankets do a surprisingly great job at reducing concussion. Don’t get me wrong – it is still loud when you’re shooting a muzzle brake, but there isn’t much concussion. I may opt to add panels on the walls and ceiling in the future – but I doubt it.

Custom Shooting Bench/Table

I designed a custom shooting bench that I can either sit at or lie down prone on top of. I fabricated and welded the base myself out of thick steel tubing. Most of the steel is 3.5” square tubing that is 3/16” thick. The top is a 2-inch thick, hard maple butcherblock that is 10 feet long and 4 feet wide. The total weight of the table is very close to 900 lbs! The table is sitting on leveling machine casters with retractable anti-vibration pads. I wanted it to be solid and not budge a fraction of an inch under recoil. I nailed that goal! This table is awesome!

The height of the table is 34 inches from the floor to the top of the bench, which is ideal for me. I debated on the bench height for a while! After a bunch of research, it seems like 32-36” bench height is the most common. I actually did all the calculations and design for the bench height before the pipe for the tunnel was even set in the ground, so I dropped the pipe a few inches below the floor of the shooter’s room so that when a rifle with a bipod is on the 34” tall bench, it would be perfectly centered inside the tunnel.

Dropping the tunnel a few inches below the basement floor seemed like a good idea in case it ever flooded, that would help keep the water from going into my basement. The target room is yet another few inches lower than the tunnel, meaning the water would pool in that room. So, if I ever did get water inside it, I could drop a pump in the target room and get all of the water out from that side. Luckily, the thing seems sealed very well because I haven’t got any water in even when it rained for days.

I mentioned that I can lay prone on the table. When I cut out the area for the seated position, I was careful to take it out in one piece. So I can slip that back on the top and have an angle iron under the butcherblock top to support it in the front, and the backrest supports it in the rear. It basically makes it like one large flat table again. I designed it where I could cut out the other side for a left-handed shooter, but for now, they’ll have to go prone. Honestly, this whole underground range is primarily for me to shoot in to do load development, experiments, and check my zero. I don’t plan to invite friends over to shoot in it. I will say that it was an extra long table, so I had plenty of space to spread out prone, but having the extra space at the end gives me a place to set rifles, gear, or I can use it as a workbench without anything being in my way to shoot. I didn’t plan that, but it’s nice!

Here are my plans for the table with dimensions:

How does it feel? What would I change if I did it again?

I honestly had never seen or heard of an underground range that was designed like this, so it felt like I was taking a few significant risks – and I wasn’t sure how they’d turn out. But I’m very happy with the results. I prefer to use muzzle brakes when I compete in rifle matches, and firing a big rifle with a muzzle brake in an enclosed space doesn’t sound great – but it honestly isn’t bad. I think the idea of the porthole through the “blast doors” into the “expansion chamber” was a huge part of that. Thanks to Scott Seigmund for that idea! The specialized acoustic blankets that absorb energy also seem to really help.

I like the size of the space and how everything came together. Virtually all of the underground rifle ranges I’ve heard about have 2’ to 4’ diameter pipe, and the 8’ diameter pipe sure seems to be useful. With that much space, I know for sure that it isn’t affecting the bullet’s flight or adding mirage effects (more on that in the original article).

I’m very glad that I opted for a professional engineering company to design the HVAC. That would have been a place that I would’ve messed up if I hadn’t done that, so thanks to all my readers who chimed in on the first post and educated me.

I do think making the primary bullet trap out of ballistic rubber blocks would have been a mistake, but I’m glad someone educated me before I placed the order. I like the bullet trap I ended up with.

The one surprise I can think of was how expensive it was to get accurate muzzle velocities in the tunnel. I knew it might end up being complex, but I was hopeful I could still use a lower-priced device. However, the professional-grade radar has a lot of advanced features that I hope to get value from at some point.

What experiments should I run?

Okay, in my first post about the tunnel, I asked my readers for ideas on how I could make it better. I’d still be up for hearing any ideas you guys might have. But here is where I’d really like your help:

Now that I have a completely controlled environment where the wind never blows, and the mirage isn’t an issue, what experiments should I run? Please leave your ideas in the comments!

I already have a bunch of ideas I’d like to try as I get time, but I’m interested in hearing what ideas you guys have. I certainly can’t promise that I’ll do them all, but I’d be very interested to listen to all of the ideas!

More Plans & Details

If you are left wondering what I did on something specific, I bet it’s covered in my original article here:  https://precisionrifleblog.com/2021/12/27/100-yard-underground-range/