Subgun Named for a Snake-Eater
The mongoose is a carnivorous mammal, a quick little bold badass impervious to most venoms that hails from Africa and south Asia. They’re known for adventurous attacks on dangerous creatures like king cobras and scorpions. This celebrated snake-eater once even graced the sides of quart cans of Mongoose Malt Liquor, complete with a cobra in the mouth.
But this article isn’t about mammals or 8-percent ABV adult beverages, but instead the new 9mm Nemo Arms pistol caliber carbine. And while it remains to be seen if this one can eat a competitor named the Scorpion, we know that’s what the folks at Nemo are gunning for.
Long before it was graced with a name, the initial concept of the Mongoose began back in the Lockdown days of 2020. The operation of the Mongoose is decidedly different from its peers, and Nemo Arms made great strides in those early days before putting it on a shelf to address other projects for a while. At SHOT Show 2024 Nemo announced this short recoil option, but it would be another year before they began shipping guns out.
More than five years in the making, we can finally take this American-born Mongoose out for a spin.
OUTSIDE & AESTHETICS
With an overall length under 14 inches when the stock or brace is folded, the Mongoose is certainly shorter than even an exceedingly small carbine — definitely well within the Personal Defense Weapon/PDW class of firearms. It’s kinda shocking how little the included bag actually is.
There’s a lot of familiar going on, and there’s a lot of different
going on, too.
The Mongoose is laid out a bit like an AR-15 with distorted dimensions, but you can also tell there’s more under the hood. Call it AR-inspired, at least in aesthetic and accessory. The necessary continuous top rail and slotted handguard, but then a clip-on muzzle cap up front. There are two muzzle caps for the Mongoose: a flat cap to run suppressed (or to have a naked barrel sticking out) or a fat compensated muzzle cap. These are very nicely machined and anodized like the rest of the gun, with spring-loaded legs to keep it captive in the upper.
The two standard AR takedown pins marry the receivers, but there’s also a mysterious third takedown pin surrounded by a gray polygon. There are lightening cuts all over, but not too aggressive. A swappable ejector is bolted in on the left side. The magwell has a generous flare to aid insertion, and there’s even a cutout that corresponds to a witness hole in the magazine.
Regarding magazines, the Nemo Arms Mongoose is compatible with CZ Scorpion EVO 3 series magazines, but it was specifically designed to feed from Magpul EV9 magazines. It makes sense because the Magpul EV9s are consistent in quality, hold 35 rounds apiece, and are under 20 bucks each. The Mongoose only comes with one, but they’re readily available. There are other options, too. Of course, CZ makes mags, also Manticore Arms, ETS Group, ProMag, Palmetto State Armory, and more. You can get them from stubby 10 rounders to 50-round drums and everything between. There are also plenty of extensions and pouches and couplers and other accessories out there.
Continuing the AR theme, the Mongoose comes standard with an ALG ACT trigger, Magpul MOE slimline grip, and a 90-degree throw ambidextrous selector. The selector does have a smaller right-side paddle, but not having it short throw feels like a missed opportunity. The trigger can be swapped out, but as Nemo Arms doesn’t yet have a definitive list of compatibility, don’t get too married to one idea.
The bolt lock/release is AR-style, except it’s also fully ambidextrous with mirrored and slaved controls. This is a beautiful piece of machining but more than mere good looks because the Mongoose locks the bolt open on an empty magazine (a feature not universal on pistol-caliber carbines).
The charging handle isn’t an AR-style (thank goodness!), but it is playfully reminiscent of it. It’s set forward, doesn’t move when you shoot, and though it isn’t ambidextrous it can easily be swapped from one side to the other.
The magazine release is straight-subgun. Paddle is at the bottom, which you can also push forward with the trigger finger. The recommendation here would be to grasp the empty magazine with your support hand, wrapping your fingers around the front while activating the release with your thumb around the back.
The rear is capped with a Picatinny rail for a brace/stock/sling swivel. At the time of writing, Nemo Arms currently sells and ships the Mongoose with a brace but you have to click through and agree to a “brace notice” on their webpage first. Stuff like that happens when the exact legality of an item has flip-flopped many times in a decade, but we’re currently in a “brace OK” time frame (subject to change by the whimsy of those in power).
The brace itself is an SB Tactical FS1913 synthetic folding brace. At best braces are kinda dodgy buttstocks anyway, but this just doesn’t match the rest of the gun. The plastic has some flex and unlocking it can be difficult. We didn’t break this one, but a tax stamp and real stock is a better idea.
ACTION & OPERATION
The outside may be pretty, but it’s what’s on the inside that counts. And when it comes to pistol-caliber carbines, there are two main buckets when it comes to action and operation: simple/straight blowback and everything else.
Simple blowback is the crudest and easiest way to make an automatic weapon in the world, which is why it’s been popular for more than a century. The only thing that holds a round in the chamber in a straight blowback firearm is the bolt and the spring, so their combined weight has to be extremely heavy in order to be safe. More mass moving back and forth means more recoil, which is harder on parts and harder on you. Everything has to be so heavy; this is only practical with pistol rounds (and even then it still can suck). But as it’s easy and cheap, companies looking for easy and cheap go this route. It’s extremely popular with 9mm carbines for this reason, but don’t be fooled into paying a premium for it.
Adding in just a little bit of delay changes the equation considerably. The idea is to keep the shell in the chamber for slightly longer, allowing the pressure to drop to a more manageable level — you just need enough energy to reliably eject the empty casing and load the next round. This is why any sort of delay equates to a lighter bolt, lighter spring, and less felt recoil.
There are a lot of ways to cut this pie. Roller delay was made famous by HK, and the Grand Power Stribog SP9A3 and Springfield Armory Kuna employ their own forms it. Maxim Defense also has a roller delay, but on the buffer instead of the bolt. CMMG uses what they call radial delay. SIG Sauer comes from a different direction by gas-operating their 9mm MPX. There’s also lever delay, chamber-ring delay, bearing delay, even experiments with magnetic delay, and more.
What makes the Nemo Mongoose unique in this field is that it uses a short recoil operation instead of any form of blowback. Specifically, the Mongoose uses a Browning tilting barrel action or tilt-barrel action, the same type that’s in almost every locked-breech pistol produced today (there are exceptions, but they are outliers).
We asked Nemo about the original inspiration for the short recoil action. The mastermind behind the Mongoose, Nemo’s VP of Engineering, Dan Neitzling, didn’t like that he could shoot a 9mm handgun all day without worry about recoil fatigue — but the same wasn’t true when it came to straight blowback guns. “So, the practical thought in my mind was to put the handgun’s operating system into a PCC …”
And indeed, the bolt and barrel on the Mongoose look just like a handgun barrel and slide but with the sides cut off.
Here’s a rough overview of how it works. Follow along with our diagram.
(1) The barrel and bolt are locked together and then move together under recoil after the round is fired. It’s a short distance (we measured this distance to be about 1/8 inch); just enough time for the chamber pressure to drop.
(2) The barrel ends its rear movement, tilts, and the breech end of the barrel travels down the sides of the bolt face to unlock.
(3) Once fully decoupled from the barrel, the bolt continues to the rear, ejecting the spent casing. (And also resets the trigger and hammer — not shown in this simple diagram.)
(4) Using spring pressure from the recoil spring, the bolt then snaps forward, strips a round from the magazine, chambers the new round, and re-locks the action.
The Nemo Arms Mongoose might be the only current production tilt-barrel PCC out there — kind of. The other tilt-barrel contenders aren’t built as carbines or PDWs outright, but instead are pistols that have been converted for the role like the CAA Roni (Glock-in-a-Box) or the Flux Defense Raider (SIG-with-a-Stock).
Because the barrel is moving as part of the operation, in theory short recoil guns will be inherently less mechanically accurate, but the practical reality is a different story — at least when it comes to pistol calibers. We’re not talking long-range rounds known for tight groups, but we can’t pretend that the barrel reciprocating is exactly aiding accuracy either. Within the envelope it’ll actually be used? Call it a wash.
DISASSEMBLY
Taking apart the Nemo Arms Mongoose is different from other guns in this category because the guts come out the front instead of the back. After clearing, the first thing you have to do is take off the muzzle cap. Though the manual doesn’t mention it, the bolt needs to be pulled back in order to accomplish this, something we discovered only after several minutes of swearing.
After locking the bolt to the rear, locate that third front takedown pin. It rotates clockwise 90 degrees and then can be pushed out to a retained position. At this point you slowly release the bolt and pull out the charging handle. If you wanted to change what side the charging handle sits, you could do that here.
Once the charging handle is removed, the barrel, bolt assembly, and charging handle carrier will slide out the front of the Mongoose. Now the upper and lower assemblies can be detached with two pins a la an AR-15.
SILENCERS
Tilt-barrels aren’t naturally easy or great to suppress; it’s just something that we’ve worked our way through because that action is so ubiquitous in pistols. Any weight you put up front makes tilting harder (or impossible) to accomplish, and this is only exacerbated by barrel length. Under most circumstances you need a booster assembly or Nielsen device to allow a suppressor to cycle on a tilt-barrel system. These devices use a spring-and-piston system to temporarily float the added weight of a silencer long enough to allow the gun to cycle, but even then it’s not a guarantee.
These days many pistols will run alright, but only because much of the homework has simply been copied from others. Unfortunately, not even all newer tilt-barrel designs take well to silencers. Early models of the SIG P320/M17 and assorted carbine conversions didn’t run well suppressed without modification to allow those tilt-barrels to unlock a bit faster, and it’s still not considered a “good” host today.
No surprise then that Nemo Arms has some size/weight recommendations when it comes to cans. Namely, no longer than 5.25 inches and not heavier than 8.5 ounces. We’re not sure if these length and weight restrictions include the booster or not, but regardless we couldn’t get the Mongoose to run reliably with any silencer we tried. We’re sure this is the reason Nemo went ahead and made a dedicated silencer for the Mongoose themselves. Named the X-NASO after the scientific name for the long-nosed mongoose, Xenogale Naso, it’s under 5 inches long and only weighs 6.6 ounces — look forward to a separate piece in the future.
AMMUNITION & ON THE RANGE
We didn’t go crazy outfitting the Mongoose, though it wore both the new Trijicon MRO SD (their best model yet!) and also an EOTech EXPS 3-0 because PDWs deserve bigassed windows.
Nemo Arms says to not use the Mongoose with +P+ ammunition. This sort of stout load is the type traditional blowback guns would love to eat all day but not tilt-barreled actions.
With this advice we went to the range. The sessions were A-OK with no failures of any kind. Though the recoil is soft, like other PDWs this size it’s almost too small to shoot comfortably — a bit like how you have to lay down to get into a sports car compared to a Camry. There’s little space for your support hand, but we found it nice to use the charging handle like a gas pedal. Add a finger index to the front and this might be the ticket. This is also the exact situation the vertical grip was invented for — if you get a tax stamp, that should be the next purchase after a real buttstock. Or just magwell grab like it’s the 1990s.
Along those lines, there’s not a ton of real estate for mounting lights either. The new SureFire XC3 upside down on top in a rhino configuration actually works very well; you’ll always be able to hit the light, but it’ll never happen by accident. Otherwise you’ll want to mount a traditional tube-style light on the opposite side and use a remote.
We couldn’t find any immediate discernible difference in felt recoil running the compensated cap versus the flat cap when swapping them back and forth. In part because there’s no front surface directing the gas, and in part because 9mm just doesn’t have the ass. A treaded brake may do better, but then you’re going to mess with the tilt-barrel system directly.
The real issues came with the vaunted Bag of Random. If you want to test if a gun will run with a variety of ammo, load up some party mags that taste the full rainbow of rounds — the Bag of Random is a continually changing and evolving sack of 9mm ammunition. It has everything from pissweak foreign ammunition that’s long been banned from importation, strong subgun rounds, modern defensive offerings, dumb discontinued designs, along with range training rounds that span the spectrum of grain weight and bullet shape. After multiple BoR failings, the testing became more formal, filling mags with known ammunition and recording the results.
The least bothersome ammunition problem were the rounds that would cycle the gun but not lock the bolt back on empty. As a functional BHO is still somewhat novel in this space, it’s a nice-to-have but not a killer. But there are loads it simply doesn’t like.
It would be nice if we could say “use 124-grain FMJ ammo” but we can’t. Blazer 124-grain cycled and locked it back, but neither Aguila or Belom 124-grain FMJ cycled it at all. Heavier and subsonic was hit and miss. Federal Syntech 150-grain would cycle and not lock back, but it didn’t like 165-grain ammo of any type. No Russian or former Soviet state ammo ran. It did like a lot of defensive rounds, especially Hydra-Shok Deep. Weird, fast, frangible Norma 65-grain training rounds were great out of this too. Some cheap 115-grain like Magtech worked well.
All told, if the ammo came from a country where you wouldn’t want to drink the tap water, it’s probably not going to run in the Mongoose either.
When we asked Nemo Arms about this, they weren’t surprised as they’ve been having difficulty nailing this down too. Some guns like ammo X, other guns hate that ammo. Because it’s a short recoil action, they suspect it might have to do with how shooters are physically holding the gun, like limp-wristing a pistol. Maybe there’s a ghost in the machine, and if the ghost thinks you’re ugly it’s not going to run.
It’s hard not to make a comparison with semiautomatic shotgun. Semiautomatic shotguns are notoriously sensitive to load, and sometimes even individual shotguns of the same type will have different preferences. But when they run — they run great and are fun as hell. The Mongoose is like that.
So as with a semiautomatic shotgun, our recommendation would be to get a bunch of different 9mm and find out what your gun likes to eat. If it runs great, tear off the end of the box and put it in the bag with your gun so you’ll remember. And never never try to run something for real work you haven’t tested before — good advice across the board but especially important here.
LOOSE ROUNDS
Blowback guns are popular because they are cheap and work well, but you don’t want to shoot one all day long, even in 9mm. It’s hard to get new things into the market instead of just clones and copies, because producing something new takes considerable time and effort. Double-so when you’re doing it in America. The engineering expenses are hard to recoup. There are no guarantees. And unless you can produce at massive scale, the costs will be high. I commend Nemo Arms for doing it the hard way. This is a small, lightweight PDW package which is fun to run, when it’s actually running.
In this day and age, it seems that a gun not running with any and every kind of ammunition should be an immediate dealbreaker, but that’s not necessarily the case — it just depends on who the market is. If it’s an issued gun, be it for a law enforcement agency or specialized military unit, then a more-limited ammunition and silencer compatibility doesn’t really matter because they’ll simply be issued the appropriate stuff. And yet, if there’s an equivalent that runs just as well, there needs to be a compelling reason for purchase, and pricewise the Mongoose sits near the very top of the PCC market.
Nemo Arms has been up front about their asterisks, but there are still asterisks to contend with. The Mongoose is this strange combination of beautiful machining and thoughtful design, with some things that just aren’t quite right. As a fan of small business, American manufacturing, and pursuit of passion, these little ghosts and dangling details break my heart a bit. No one has ever made a tilt-barrel pistol-caliber carbine like this before, perhaps now we know why.
I remain hopeful for version 2.0.
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