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The Airsoft M4 platform enjoys a level of customization and aftermarket support that is unmatched by any other design out there; which also makes it a pretty good place to start building an "ultimate" version from too.
A lot of folks have vastly different ideas of what their ultimate M4 would be. For some, it's a HPA engine with a hair trigger and 40 RPS. For others, it may be a near identical copy of an issue Mk.18 down to sourcing period correct accessories. For myself, I based my build around a few criteria that would dictate some of the final results:
The rifle must be lightweight. Excess ounces = more pain at the end of the day.
Barrel length would not exceed a 14.5" envelope. That also means no suppressor (which in turn means more weight savings and better handling).
I wanted a low profile rail setup. No excess picatinny rails. This ultimately meant no PEQ box for the battery (I hate PEQ's too).
Due to the above requirement, this also meant rear-wired, so the battery had to fit inside the buffer tube. Rear wire opens up the possibility for easily swapped upper receivers too.
Long term durability was important, so my internal parts choice is based around parts that I have found to have outstanding durability.
Beauty is in the eye of the beholder, of course, but it had to at least be aesthetically pleasing to my eye.
Internal gearbox setup and parts list:
Prometheus 6.03 407mm stainless steel barrel
Pro-Win hop up chamber w/M-Nub
Prometheus Purple bucking / R-Hopped
Retro Arms CNC 8mm QCS CNC gearbox
Lonex Extreme selector plate
VFC safety lever
8mm F.L.T. CNC steel bushings
Guarder precision oil tampered air nozzle
Lonex Extreme tappet plate
Guarder double o-ring stainless steel cylinder head
Guarder super lucid chromium plated cylinder
3/16" 70 degree Air-Pad
Retro Arms CNC POM double o-ring ported piston head
SHS 14 tooth full metal rack piston / AOE corrected
KWA RM4 gearset and anti-reversal latch, short-stroked 1 tooth
Prometheus M120 irregular pitch spring
SHS cutoff lever
SHS trigger switch
VFC trigger
Modify steel shims
Lonex A2 high torque motor
Deans plug
For the heart of the gun, I chose a Retro Arms CNC 8mm QCS gearbox.
These are literally the strongest gearboxes you can get on the market.
The Quick Change Spring feature is a nice option to have too, and allows
you to swap springs fairly fast (even faster should you get lucky with a
receiver set that allows for removal of the spring without taking out
the gearbox). They also have generous clearances for precision shimming
of your chosen gears.
I chose the RM4 gearset because I have found it to be a very tough set of gears - easily capable of handling high ROF and heavy spring builds. I've yet to see a set of these gears cross my bench that was broken for that matter. Since I already had these on hand, it also meant $100 saved over a set of Seigetek's that I could place towards other items.
The SHS trigger switch is just acting as a functional placeholder until the GATE Titan MOSFET eventually becomes available.
I discovered a couple of things when putting this setup together, one of which was it had ungodly compression - the M120 was delivering over 325+ FPS with a .20g BB, secondly, it had a tendency to double fire in the semi-auto position (overspin) unless you were very quick on the trigger. Short-stroking the piston and sector gear 1 tooth respectively solved this problem immediately and brought the FPS down to 398 w/.20g BB's. This also has the side-effect of boosting the ROF slightly.
SHS 14 tooth piston. This version comes from the factory pre-lightened and with the second to last tooth removed. I have short-stroked the first tooth on the rack.
Completed gearbox. Ready for lube!
Barrel-wise, I went with a Prometheus 6.03 407mm stainless steel setup. Prommy barrels are my preferred choice for a good mid-range do-it-all barrel that will handle a wide variety of BB brands. There are certainly 'better' setups, like Edgi as one example, but I kind of feel those are overkill for anything but a dedicated DMR or sniper platform where eking out every bit of accuracy is vitally important. For a full-auto BB hose, a Prometheus does the job quite well. For added range, an R-Hop is a must, and I have installed one with a modified Prometheus Purple bucking in conjunction with an M-Nub.
I had to go with a Pro-Win chamber vs. a Lonex chamber due to the fact that the Lonex would not seat correctly with my outer barrel. At least, not without some minor milling to make it fit. Until I get a mill up and running again, the Pro-Win will do the job. Personally, I find either chamber design acceptable.
Hop up chamber, modified Prometheus purple bucking, R-Hop patch, and inner barrel.
External setup and parts used
I started this build idea off with a just a spare VFC receiver set that was gathering dust. While these do not have real steel style markings, my
primary concern was using a receiver set that would require as little
fitting as possible with the rest of the goods. My experience has been
that VFC receivers are generally among the best in this regard. Unlike Madbull,
Lonex, Hurricane and G&P receivers I have used in the past that often require
varying levels of hand fitting from minor to extreme to drop in custom gearboxes,
accessories, etc. VFC seems to have none of those problems. Things just
fit very well with them.
Full externals list:
VFC receiver set
VFC magazine release
VFC charging handle, dust cover, bolt cover, and pins
Madbull Daniel Defense 14.5" Government profile outer barrel
PTS Griffin Armament M4SDII Flash / Compensator
Troy style rear BUIS / KAC style front BUIS (can't remember the brands)
Surefire M952 light w/Malkoff Devices 450 lumen LED
Haley Strategic Thorntail light mount
Fortis QD sling socket
A local shop was offering a killer deal on Cerakoting services, and as I had several other customers guns in their queue, I decided to get the receivers and rail done up for this project. Honestly the hardest part about Cerakoting something is decided which color to choose! Eventually I settled on what they call "Tungsten" which is an interesting color that seems to change shade and tone depending on the lighting. It also happens to look good with both black and FDE accents.
Keeping the lightweight meme in mind, I went with the Mission First Tactical Minimalist buttstock in their "scorched dark earth" color - which is really just FDE by any other name. This stock weighs only 5.8oz and is one of my top picks for M4 style stocks.
I chose the G&P buffer tube explicitly for its features - it's one the lightest tubes on the market, in addition to offering a real castle nut for dual tightening (as well as the usual inside the tube screw) and it also has the most internal room of any tube I have tried. As such, your typical tri-panel or even regular ol' 11.1v buffer tube LiPo's will fit easily with room to spare. The top end has some nice laser etched numbered positions that are visible through the witness hole on the top of the Mission First Tactical Minimalist stock as well.
The Madbull Troy CPG motor grip is another one of my favorite M4 accessories. It has the same angle and shape as the standard A2 style, but with improved texture and none of that irksome finger nub. The motor plate is nicely ventilated which aids in keeping your motor cool. Color-wise, this matches the finish of the MFT stock and the CMR panels perfectly.
The PTS Centurion Arms CMR rail is a fantastic piece of kit, and by far
my favorite low-profile rail. It's slim, lightweight, and doesn't add
any unnecessary rails. The panel kit is perhaps the best feature, and
the panels themselves have a very grippy golf ball-like texture. Some
guys like to run vertical foregrips or angled foregrips on their rails,
on this setup I chose to keep it low profile and run it with just the
handstop as an indexing point.
Unfortunately the QD sockets on this rail are far too tight for any of the real steel QD sling loops I had on hand to fit, so I have had to resort to using a Fortis QD socket for this purpose.
I'm running the Surefire tape switch at the 12 o'clock position for ambidextrous use, though I suspect I will replace this with the Surefire SR switch in the future for a cleaner setup.
Underside of rail.
Since I was using a 407mm length inner barrel, this stuck into the flash hider area a good amount. The exit hole on the flash hider was too small to allow the barrel to pass through, so I bored it out just enough to where it fits semi-snug and provides a rigid support for the muzzle end of the inner barrel.
Finishing off the build was a Blue Force Gear VCAS sling and some PTS EPM magazines with Magpul ranger pulls. I'm still deciding on the optics, but right now it seems to transition between an Aimpoint T2 in a Scalarworks mount or an Aimpoint CompM68 in a Knights Armament mount.
This gun will do an crisp 28 RPS with an 11.1v 1200mah LiPo. Total weight of the package is 7.1 lbs. Not bad at all.
Overall, this is a well balanced, lightweight and snappy build, and I'm very happy with the final result - it was also a lot of fun to build too!
