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S3F Solutions MK-601

One of the allures of the AR15 design is the relative ease to which you can swap out the upper receiver.  Having a quality lower receiver is important, but for most purposes you only need one and then it’s as easy as pushing two pins, maybe changing out the buffer spring and buffer and suddenly you have repurposed your rifle.

Because the lower is considered the firearm for legal purposes, there is usually no issue whatsoever with ordering an upper receiver and having it sent right to your house.  Yes, quality in the lower is important, quality in the upper receiver slightly more so.  The lower is very negotiable; as long as it’s “in spec” all manner of improved controls can make a $60 lower run just as well as a designer lower from a big name manufacturer.

S3F Solutions MK 601

In the case of upper receivers, it’s a little more complicated.  The receiver and barrel are the heart, their interaction with and the fit of the bolt carrier group will decide much of the rifles performance, especially when it comes to putting bullets where you want them to go and doing so consistently and reliably.  Of course there are a few aftermarket upgrades that can improve an upper, though for the most part if the heart doesn’t work, no manner of rail system, compensator or engraved ejection port is going to fix that.  I like the idea of being able to buy a complete upper as opposed to shelling out for a complete rifle because it lets me get the gun build rolling sooner; though to be honest I’ve never purchased a complete upper because I prefer to build my own so when S3F Solutions contacted me about doing a review on their MK 601 complete upper receiver, I was genuinely entering uncharted territory.  Not counting other rifles I have reviewed, all of my personal ARs have been built from the ground up.

S3F Solutions MK 601

So I get a box in the mail and it contains a hydro dipped MK 601 in the Battle Worn finish, which ships with the popular Raptor charging handle, a 13.5” Arsenal Democracy rail, buffer spring and 100g buffer, and a chrome lined microslick bolt carrier group.  S3F offers many barrel device options for the MK 601, the test upper came equipped with the Templar Tactical Suppressor Mount flash hider.

First thing first, I pulled pins on four lowers and checked to see how well the MK 601 mated to different manufacturers lowers. Aside from an almost too tight fit on a Battle Born lower and a small amount of play on an Umbrella Corporation lower, the MK 601 fit well and did not have an unacceptable level of movement at all (zero movement in the case of a Seekins Precision and Black Rain).  For the pew portion of the test I went with my Seekins lower; B5 Systems stock and a Geissele SSA-E trigger.  The MK 601 doesn’t ship with sights or optics, so that was up to me.  Troy BUIS, Aimpoint Micro, a Unity Tactical keymod VFG and keymod Rail Scales panels to protect my paws from heat and I was good to go.

S3F Solutions MK 601

As usual, I would be testing for extreme reliability, consistency and accuracy.  With the upper pre-treated with FIREClean, I went to work on a 1000 round test that included Wolf Polyformance 55 gr, Fiocchi 55 gr and Lake City SS109 out of Magpul, Surefire and assorted aluminum body USGI magazines.

The MK 601 may be the quickest zero I have ever done; using SS109 I laid in for a 50 meter zero and was done in 10 rounds (with 5 to confirm). One click to the left on the rear, one full turn down on the front sight.  I slaved the Aimpoint to the irons, folded them down and confirmed my red dot zero then began the work of trying to run the upper to death.

My first dislike was immediately apparent; the 13.5” rail has a permanent section of picatinny at the end.  Depending on your hand placement and/or grip option, this piece of rail may give you an uneven grip surface as it did me.  If the rail was a 15” or longer this wouldn’t have been a problem, though the 13.5” Arsenal Democracy is at that length that the average shooter will encounter that section of rail in their grip using a handstop or VFG with a fully extended arm.

Now, this is a personal preference issue but for me it doesn’t make much sense to slap on one piece of rail space under the rail on an otherwise keymod design.  In the end it’s a minor issue but one worth bringing up.  The rail is strong, built in anti-rotation tabs keep it from twisting out of alignment with the receiver and the body has no noticeable flex during shooting, even in unorthodox positions such as off-axis where I have seen other rails flex POA/POI.

S3F Solutions MK 601

As to shooting the MK 601, the very first thing you will notice is its weight.  It’s on the heavier side of what might be considered an average upper weight, breaking at just over 5 pounds without optics/sights/etc.  It doesn’t make the rifle top heavy or unbalanced, though you will respect it more when staying on the gun through multiple magazines and perhaps think about upping your upper body routine when making target transitions.

The extra weight helps greatly with keeping the gun flat; muzzle rise is not an issue for practical accuracy, nor is barrel dip or any horizontal sway.  I found the recoil pulse manageable and user friendly in a number of positions.  The MK 601 has a 16 position adjustable gas block for those wishing to tune the firing cycle and smooth it out to their preferred sweet spot but the factory adjustment worked just fine for me.

I ran the MK 601 as hot as I could, which was pretty hot.  The rail doesn’t pick up heat fast, as there is a fair amount of room between it and the barrel.  When it does start to collect heat it goes from uncomfortable to unbearable fairly quickly though that’s more an issue with the First Law of Thermodynamics than it is the upper design.  The most important thing to note about the upper when hot is that accuracy did not seem to be affected in any meaningful way.

From fist sized rapid fire groups at 25 yards to prone unsupported 10×10 steel plate strikes at 300 meters, the MK601 more than did its part when I was doing mine.  This is largely thanks to the .223 Wylde chamber and 1:8 416R stainless steel ratchet-rifled barrel (I personally prefer ratchet rifling over polygonal so I am a bit prejudicial there but the accuracy speaks for itself).  Lock up with the MK 601 BCG was precise every time, even as fouling built up during shooting.

Over the course of 1000 rounds I experienced zero malfunctions, ammunition or rifle caused.  From slow fire to running the trigger as fast as I could, the MK 601 continued to shoot bullets where I wanted them to go with indifference to my attempts to keep it from performing.  Running on the high end of operating temperatures there was a noticeable change in accuracy, though I only noticed this when shooting out to 200 meters in the prone, unsupported in 90 degree heat…so it was as much me as it was the rifle and the accuracy difference was with an inch variance.  The final accuracy test speaks for itself; 5 round group at 50 meters on an ambient temperature gun with ambient temperature SS109 ammo fired from bipod-supported prone.

S3F Solutions MK 601 50M group

S3F Solutions MK 601 50M group

My final opinion is that the MK 601 deserves a home among higher-end complete upper receivers.  With its lineage linked directly to the Arsenal Democracy Reaper 33, I expected excellent performance before I fired a single round as was not disappointed after 1000.  S3F offers the MK 601 with multiple Cerakote/hydo dip color options, as well as muzzle devices (Sufrefire/AAC in addition to the Templar Tactical) in lengths from 14.5” to 18” and available in 5.56 or 300BLK, anyone looking to purchase a complete high-end upper should seriously consider the MK601.


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