Sons of Liberty Gun Works Trunk Monkey and M4-89 AR-15s: Review

Disclaimer!

This article was orginally submitted before the 2023 ATF stabalizing brace policy ruling. The company featured in this article may or may not still offer the pistol stabalizing brace supplied with the reviewed firearm.

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Anyone properly educated on American history knows who the Sons of Liberty were. They were a group of patriots who first rebelled against the British Stamp Act, and then were instrumental in the American Revolution. Their members included Samuel Adams, John Hancock, Paul Revere, and Patrick Henry. The Sons of Liberty tarred and feathered British colonial government officials and organized the Boston Tea Party. They were involved in arson against loyalists. And they were heavily involved in the struggle for American independence when the war finally arrived.

Which meant, in the eyes of the British, they were seditionists. Traitors. Criminals. So, anyone with a knowledge of history naming their company Sons of Liberty is taking a very definite stand. Sons of Liberty Gun Works (known everywhere as SOLGW) has been very active in pro­Second Amendment causes. Supporting pro­gun politicians. Founder Mike Mihalski was even on the ground in Kenosha when the Kyle Rittenhouse verdict was announced. They’re about as pro­gun, pro­freedom, and pro-America as a gun company gets. As Mihalski says about starting SOLGW, “It wasn’t just a rifle company, it was a cause.”

Which, honestly, wouldn’t matter much if their products were sub-par. Instead, SOLGW rifles and pistols have earned an envious reputation in the industry as well-made, hard-use firearms. For this article I secured both a rifle and a pistol — SOLGW’s flagship carbine, their M4-89, and their Trunk Monkey, perhaps their most well-known AR pistol. And beyond barrel length, the two guns are quite different, and show a breadth and depth of understanding of the AR platform that is, unfortunately, quite uncommon among AR manufacturers.

Trunk Monkey

When it comes to great gun names, I think SOLGW’s Trunk Monkey beats out Q’s Honey Badger. And it is a great product to go along with that great name. Why “Trunk Monkey?” Think Truck Gun, if that helps. And remember — the odds of you needing a truck gun are low, but never zero. The Trunk Monkey is a 10.5-inch barreled hard-use AR pistol that is about as close to perfect as you’re going to find, perfect for a truck gun, home defense, entry team, or just having fun at the range — and priced right. Technically it is a “special edition” of SOLGW’s M4 Patrol Rifle, which like all SOLGW’s ARs uses a standard direct gas impingement operating system.

Mihalski wanted to build hard-use rifles, “fighting rifles,” and even though it’s “just” a pistol the Trunk Monkey embraces that philosophy. First, it starts with a 10.5-inch 4150 Chromoly Vanadium steel barrel that is MPI and HPT tested. A brief word about that — in a lot of ARs you’ll see parts that are advertised as being MPI (magnetic particle inspected) and HPT (high pressure testing) tested. Usually they are just batch tested — one part per batch is tested, if that. With SOLGW guns, every part with that designation (barrels and bolts) has been individually MPI and HPT tested.

The barrel has a 5.56 NATO chamber, 1:7­inch twist rate rifling, and a QPQ corrosion resistant finish inside and out. Also known as Tenifer or Melonite, QPQ (quench-polish-quench) provides a finish that protects against corrosion and wear better than hard chrome or nickel plating. The barrel is threaded 1/2-28 and tipped with a standard A2 flash hider. The barrel is equipped with a fixed front sight post (technically an FSB, front sight base), which is not so common these days. It is paired with a Magpul MBUS flip-up rear sight. While this front sight is always ready, that’s not the best thing about it — the best thing about it is that it is pinned in place per Mil-Spec.

If an AR­15 has reliability issues, there is a 99% chance it will be due to either the gas system not getting enough gas, or the bolt/carrier being improperly machined or constructed. It is possible for gas blocks to migrate, especially if the barrel is not dimpled for the gas block set screws. One sure way to eliminate this problem entirely is to use a gas block that is pinned in place, per the original specs. Per the original specs, the FSB of the Trunk Monkey, which also serves as the gas block, is pinned in place. The FSB of the Trunk Monkey sits over a gas port sized specifically for this gun, 0.073 of an inch. Check out the sidebar for specific SOLGW specs, but as for a brief explanation of why they went with this port size.

The Trunk Monkey mimics the military’s Mk18 in barrel length. The Mk18 has a 10.5-inch barrel with a 0.070" gas port and an H2 buffer. However, that military gun is built to run on hotter military 5.56 NATO ammunition. The folks at SOLGW build all of their guns to run with hot 5.56 ammo, soft commercial ammo, unsuppressed as well as suppressed. As a result, they went with a slightly larger 0.073" gas port on this gun, paired with an H2 buffer. The barrel and the base of the front sight base are enclosed by a handguard you don’t see that often, the Magpul SL. This is a fabulous handguard, with a roughly oval profile, and superior to the original polymer CAR­15 or M4 handguards. Why? The length. Yes, this handguard has metal heat shields, but what makes it superior to the military handguard is how it stretches forward and encloses the gas port and the base of the front sight.

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Not only does this give you approximately two inches more handguard to grab onto (overall it’s 8.9­inches long), it does it in a way that protects your hand from the hottest part of the gun (the gas block). If you look close, you’ll see the front portion of the handguard flares out a little to give the gas block a little more room to breathe. Shoot enough, and the entire front sight tower will get to hot to touch. This handguard does whatever it can to keep your fingers away from that heat. It also sports M­LOK slots for attachments such as lights, slings, and hand stops. Just a reminder, hand stops and angled foregrips are legal on pistols according to federal law, vertical foregrips are not. No, this doesn’t make sense, nor is it Constitutional, but almost no federal gun law is. As the Sons of Liberty would tell you, they wrote the Second Amendment to guarantee you continued access to weapons of war, in case your government (once again) drifted toward tyranny. The entire Bill of Rights is a restraining order against our own government. And the Sons of Liberty went to war over far less onerous encroachments on their freedom than we currently suffer. Everybody clear on that? Good. Moving on … .

Both the upper and lower receiver are standard in design, 7075­T6 aluminum Cerra Forge forgings and anodized to Mil-Spec. That said, the fit between the upper and lower receiver is the best, with the smallest gap, that I’ve ever seen outside of a custom AR­15. Snug, with no play, but not so tight you need a punch to pop out the receiver pins. And both the Trunk Monkey and the M4-89 exhibited the same great fit. It wouldn’t surprise me if the upper and lower receivers were machined together as a set. The lower receiver sports SOLGW’s “Angry Patriot” logo, a skull in a tricornered hat in front of crossed muskets and the date 1776, and pictogram markings for the selector. The lower receiver has an oversize aluminum trigger guard that you can quickly pivot down if you’re using bulky winter gloves. Externally the magazine well looks identical to GI guns, but internally it sports a more aggressive bevel to help smooth reloads.

Inside the lower receiver is SOLGW’s Liberty Fighting Trigger, which uses standard configuration parts with improved surfaces that have then been given a nickel-boron coating. This trigger is advertised as providing a consistent 6.0 to 6.5­pound single-stage trigger pull. Trigger pull on my sample was 5.5 pounds., perfectly acceptable for a duty/hard use rifle. In fact, both the LFT in the Trunk Monkey and in the M4­89 provided 5.5­pound trigger pulls. When testing them side by side, the trigger on the Trunk Monkey was marginally crisper. The pistol grip is a P23 from B5 Systems. It has a more vertical angle than the military A2, with aggressive texturing. I don’t care for it personally, simply because it doesn’t have an internal compartment, and inside the pistol grip is where I like to store spare batteries for my optic. But that’s literally my only complaint.

Selector, magazine release, and bolt catch are all Mil-Spec. The charging handle is a bit oversize, SOLGW’s Mod 4, which has bilateral, aggressively serrated levers. Mihalski says that the BCG is the heartbeat of the gun, second only in importance to the barrel, especially in endurance/fighting guns. He believes bolt/extractor/key material and tolerances are far more important than coatings. Their bolts are true MilSpec 158 Carpenter steel. Each bolt is individually high­pressure proof load tested, and each gun is test fired in the shop twice before shipping out.

Their gas keys have Grade A fasteners that are properly staked. They use a five-coil extra power extractor spring with black insert to guarantee extraction. They use phosphate coatings for their BCGs simply because phosphate holds lube really well. In fact, check out the sidebar, where you can see all of the specs for SOLGW’s bolts and carriers. And I mean ALLLL the specs. Behind the bolt is an H2 buffer, and it resides inside a standard carbine buffer tube. On the outside of the tube is an SB Tactical SBA3 pistol arm brace. This brace has a rubber cuff at the end with a tensioning strap for your forearm. It is five-position adjustable. Overall, with the brace collapsed, this pistol is 26.25­inches long. Extending the brace adds 2.5­inches to that length. Unloaded, it weighs five pounds 10 ounces.

Sons of Liberty M4­89

The M4-89 is SOLGW’s flagship rifle, but there are a number of different models. The version I tested was the 13.7 Mid Gas LFT model, which has some unique features. SOLGW M4s are AR­15-size rifles with the standard direct impingement operating system, offered in 5.56 NATO, 300 Blackout, and 6.5 Grendel. The M4-89 guns sport SOLGW’s new M89 handguard. It is designed to be an improvement over their previous M76 handguard, which will still be available. Their rifles and pistols are available in various barrel lengths.

16-inches is of course the standard length for carbines, and the legal limit for rifle barrels without drifting into SBR territory, which requires additional federal paperwork. The sample shipped to me combines a 13.7-inch barrel with SOLGW’s NOX muzzle device permanently attached, and the end result is 16.1­inches long. If you’re confused, and this is new to you, let me explain: if the muzzle device is permanently attached it is considered part of the barrel, so pinning and welding (as the government requires) a muzzle device is a valid option if you want a shorter barrel in combination with a longer muzzle device for a total length as close to the minimum 16 inches as possible. The “combat grade” barrel on this rifle has a 5.56 NATO chamber, 1:7 twist, and is constructed of 4150 CrMoV (chromoly vanadium) steel (per Mil­Spec MIL-B­11595E). The barrel has a mid­length gas system, a QPQ finish, and a relatively lightweight “government”-type profile.

The low­profile gas block has two set screws that mate with dimples made in the barrel. They are Loctited into place. All of SOLGW’s gas blocks have a small dimple on the side to show you where to drill if you wanted a gunsmith to pin the block in place. The NOX muzzle device at the end of the barrel is a combination flash hider/muzzle brake. It is built similarly to and somewhat reminiscent of the Surefire WarComp, and these two designs are the only flash hider/brake combo units I’ve ever tested that work as advertised, both suppressing flash and providing some recoil reduction. Most combo flash hider/brake units only function as muzzle brakes, and actually increase flash over the military A2 flash hider, which is the standard. The NOX is constructed of stainless steel, with a Melonite finish.

The NOX is a three-prong unit, with the larger prong at the bottom, to keep from kicking up dust if you’re shooting prone. Just behind the prongs are two sets of three tiny ports, one set at 12 o’clock, and one set at 3 o’clock (when set up for a right-hander). While shooting I found that muzzle flash was not increased over an A2, and in fact it might have been reduced a bit, while muzzle rise was pretty much eliminated. I mean, ARs have little recoil to begin with, but this rifle didn’t climb during rapid fire, rather it just bounced around a bit. The NOX also serves as a suppressor mount for Dead Air Sandman and Nomad sound suppressors. And on that note — Mike Mihalski says, “Our approach to building guns is every rifle begins its life as a barrel, and we build the rest of the platform to support that barrel. At the time we started, and maybe still today, gas port sizes are really overlooked. Understanding gassing, and managing carrier speed, was one of the things that separated performance brands. Every single rifle we build, we anticipate it will be suppressed at some point. So, understanding gassing, and buffering, whether you’re shooting suppressed or using anemic ammo, is important.”

The barrel free floats inside SOLGW’s newer M89 handguard. This handguard is 13­inches long, and features SOLGW’s “drive­lock” design. This uses three wedges which tighten against grooves in the proprietary barrel nut, clamping it tightly. The handguard is 1.6-inches wide and 2.0­inches tall, including the MIL­STD­1913 rail which runs across the top, the slots T­marked.

While it’s rare to find an AR handguard that doesn’t work to, um, guard your hand from the barrel, not all handguards are created equal, although the differences are missed by most people. The M89, truthfully, doesn’t look like much in photos. In person is another story, it’s far more impressive in both look and feel. The M89 is the stiffest rail I’ve seen outside of Geissele SMR and Daniel Defense RIS II handguards, which is saying something. There are M­LOK slots at 3, 6, and 9 o’clock, and it comes with a QD sling swivel socket mounted on the left side. Total unloaded weight of the rifle is six pounds, 9.5 ounces.

This rifle wears one of SOLGW’s Ambi Selectors, which has a shorter lever on the right side. Not only that, but this is also a short­throw selector, requiring you to only move the lever 45­degrees to go from safe to fire instead of 90. I’ve been aware of short-throw selectors, but it wasn’t until I got hands on this rifle that I’ve ever used one. Going from Safe to Fire isn’t much of a change, although with the reduced angle you won’t have the right­side lever poking your trigger finger, no matter how long it is. However, I found that when going from Fire to Safe I didn’t have to reposition my thumb to reach the lever and pull it back. Which is objectively an improvement over the original design. I understand why people like them, but after over three decades with 90-degree safeties I still prefer those — muscle memory, or whatever. If you start out with 45­degree safeties, I have no doubt you’d prefer them.

At the rear of the rifle you’ll see a Vltor A5­style buffer tube, with a Vltor A5-H2 buffer inside. Mihalski believes it is a legitimate improvement to the standard Mil-Spec carbine buffer system. Vltor’s A5, both buffer and tube, are longer in length than what you’ll see with a standard carbine. Specifically developed at the request of the Marine Corps, the buffer has a biased spring inside keeping the moving weights in the same place at the start of each cycle. It uses a rifle­length buffer spring, which is more forgiving to ammunition types/suppressor use, etc. The buffer tube is about an inch longer, and instead of six-position adjustable, like a standard carbine, your stock on this tube is now 9-position adjustable.

The castle nut is staked in not one, not two, but three places — and the same is true of the castle nut on the Trunk Monkey. If nothing else communicates SOLGW’s commitment to producing rifles meant for hard use, it’s a castle nut staked in three spots — I don’t know anyone else who does this. Riding on the buffer tube is a Bravo stock from B5 Systems. It has a rubber buttpad and several options for attaching slings.

Range Time

I mounted the new Primary Arms PLx 1­8X scope atop the M4-89. This LPVO is getting a lot of attention simply because of its size — at 9.3-inches long and 16.5 ounces, it is shorter and lighter than most competing 1-8X scopes, sometimes by a not-insubstantial margin. The only thing close is the Nightforce NX8, which is a bit shorter, an ounce heavier, has an inferior reticle (in my opinion), and is priced nearly $500 higher. The illuminated inverted horseshoe reticle of the PLx works well at 1X at speed, and 8X magnification is more than enough to identify and engage targets out to the practical limits of the rifle.

Even before I added a pound of optic to it, there just wasn’t muzzle rise with this rifle. The muzzle moved a bit while shooting as the carrier cycled, but it didn’t want to climb. I ran everything from hot Winchester M855 5.56 NATO green tip to soft Wolf Gold .223 ammunition through it, and the rifle ate it all flawlessly. It really was a joy to shoot, and I did some speed and transition drills after the requisite accuracy testing. And as far as accuracy, with good ammo this rifle would do MOA or better with most ammunition. Three magazines are supplied with every AR, and the ones which came with both the Trunk Monkey and the M4­86 were stainless­steel 30­rounders (I think from DuraMag) with non­tilt followers and a black finish.

You can find people who will argue about anything and everything in regards to the AR­15, but especially the “right” barrel length. However, if you’re looking for the absolutely shortest rifle you can get without filing SBR paperwork, you’ll need something between a 13.7- and 14.5-inch barrel with a permanently attached muzzle device to get the overall length to the 16-inch minimum. Most modern defensive rounds for the .223/5.56 are designed to perform at the velocities those barrel lengths provide. In fact, the velocity you get with a 13.7-inch barrel at 100+ yards is about what you get with a 10.5-inch barrel at the muzzle. Which leads us back to the Trunk Monkey … .

I love hard use AR pistols like the Trunk Monkey. They are exactly the kind of firearms the Founders had in mind when they wrote the Second Amendment, and are great for personal or home defense in addition to urban combat (just like the military’s very similar Mk18). On two different range trips I got the Trunk Monkey quite warm, running between cover and engaging various paper targets, practicing my snap sight pictures, and working the Monkey off a rifle barricade. Its abbreviated length makes it quite maneuverable, and the balance is great. For as short as it is the Monkey is not as loud as you might expect, because it doesn’t have a muzzle brake, and like the M4-89, there’s no real muzzle rise, the muzzle just moves around during recoil. Mostly recoil is back, not up.

For testing I mounted the new Holosun AEMS optic atop the pistol. I believe this optic, along with the SIG ROMEO8T, has put the final nail in the coffin of the EOTech HWS, and check back in these pages for a full review of it. In short, it is a closed sight with a square window, with a 2 MOA dot/65 MOA circle reticle like the EOTech HWS, and built to military specs — completely submersible, battery life listed in years, built-in lens covers, etc. The big window and big reticle on the optic are a great choice for an AR pistol.

However, while the provided mount for the AEMS works, I don’t like the looks of it (completely unimportant) and it is not QD (which is important). If and/or when your optic dies, you need to quickly be able to use your iron sights (which are an absolute necessity on a fighting AR). Sure, you can use them through the window of the AEMS, but what if the window gets so badly damaged you can’t see through it? Excrement happens. I mean, isn’t that the whole philosophy behind having a trunk gun in the first place?

As this optic is so new, and uses a proprietary mounting pattern very similar to an oversize RMR footprint complete with forward recoil lugs in the base, only American Defense Manufacturing was making aftermarket mounts at the time I needed one. They offered them in various heights, with their QD lever, and I secured one which puts the reticle 1.93 inches over the rail. This height allows you to use night vision with the optic, and places the reticle high enough that the front sight isn’t in the way.

This optic was great for both-eyes-open running and gunning, and you don’t even notice the weight. I spent a lot of time shooting on the move, working on my transitions, and trying to improve my speed getting my first shot on target from a ready position. I used the Primary Arms 1-8X for testing, just to give the pistol every advantage. In fact, shorter barrels are often more accurate than longer ones, simply because they’re stiffer. The Trunk Monkey was nearly as accurate as the M4-89.

When they were smaller, you could order a rifle or pistol directly from SOLGW with the specific features you wanted in the build. Now that they’re larger, and demand has increased, they’re currently only offering specific standard models, with only a few variations available (barrel length, etc.). Once they’ve caught up to demand they hope to return to offering individual builds. Right now, their guns are available from a number of dealers and national distributors, including Brownells, Big Tex Outdoors, Rainier Arms, and Primary Arms. With every firearm you get a warranty certificate listing the date of manufacture, signed by the armorer who built it.

Oh, and that warranty? It covers pretty much everything, including normal wear. Not just that, SOLGW will replace the rifle if you use it in a justified shooting (as the authorities will take your AR as evidence and you’ll likely never get it back). It’s not called the “Rittenhouse clause” in their warranty, but it should be.

Sons of Liberty Gun Works Trunk Monkey

• Caliber: 5.56mm NATO
• Weight: 5 lbs., 10 oz. 
• Overall Length: 28.75 inches (brace extended) 26.5 inches (brace collapsed)
• Upper: Forged 7075 T6 aluminum
• Lower: Forged 7075 T6 aluminum
• Barrel: 10.5", 1:7 Twist, carbine-length gas system
• Muzzle Device: A2 Flash Hider
• Sights: Fixed front, Magpul MBUS rear
• Trigger: SOLGW Liberty Fighting Trigger, 5.5 pounds (tested)
• Brace: SB Tacical SBA3
• Pistol Grip: B5 Systems P23
• Handguard: Magpul SL
• MSRP: $1,249
• Manufacturer: Sons of Liberty Gun Works

Specs Son of Liberty Gun Works M4-89 Specs

• Caliber: 5.56mm NATO
• Weight: 6 lbs., 9.5 oz.
• Overall Length: 35.75 inches (stock extended) 32.25 inches (stock collapsed)
• Upper: Forged 7075 T6 aluminum
• Lower: Forged 7075 T6 aluminum
• Barrel: 13.7" Lightweight, 1:7 Twist, mid-length gas system
• Muzzle Device: SOLGW NOX flash suppressor/ muzzle brake
• Sights: None
• Trigger: SOLGW Liberty Fighting Trigger, 5.5 pounds (as tested)
• Stock: B5 Systems Bravo
• Pistol Grip: B5 Systems P23
• Handguard: 13" SOLGW M89 M-LOK
• MSRP: $1,800
• Manufacturer: Sons of Liberty Gun Works

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About the Author

James Tarr is a longtime contributor to Firearms News and other firearms publications. A former police officer he is a USPSA Production Division Grand Master. He is also the author of several books, including CARNIVORE, which was featured on The O’Reilly Factor. His current best-selling novel, Dogsoldiers, is available now through Amazon.

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