Review Request: King Arms M4

I recently was asked this question through Airsoft Gear’s forums. I thought I’d share my answer publicly with you guys, as I’ve really been quite a fan of the cost-to-value that you get from the King Arms M4.

Both of my KA M4’s came through Airsoft Gear, as well as the majority of the Guarder internals that I put into them.

The following are quoted PM’s sent between me and a friend who is interested in buying a KA M4 (edited for clarity):

Hey Juicy,

I saw on your blog, the overhoppers, that you had a King Arms M4. I’ve recently seen the gun for sale, for pretty cheap and I’m highly interested in picking one up and upgrading it.

I was just curious what your experience has been with the gun, pros and cons.

Here’s the link to the King Arms M4

I expect at such a discounted price I’ll have to upgrade which I’m more than happy to do.

Any insight would be helpful.


Yup. I’ve got two KA M4’s, actually – one CQB, one A1.


  • They’ve got Colt trades, the receiver is a little lighter shade of grey (not quite Colt grey), AEG milspec buffer tubes are pretty much bang on in sizing (fit VFC/PTS stocks great).
  • Externals are pretty much plain M4A1’s though, kind of boring until you start slapping on stuff.
  • Out of box they’re wired to the front, but I’ve routed/added wiring so that my wiring in both my KA M4’s to the rear (stock tube end cap comes off with a hex key).
  • Internally you’re looking at good gears, good piston (when you’re running a spring below 400 fps on 0.20g’s on semi), 7mm bearings (super smooth if shimmed correctly), reinforced V2 shell, single o-ring nozzle.
  • Excellent hop up unit – one of the best, definitely better than it costs in comparison to other options out there.
  • Barrel is 6.08 but a very consistent 6.08 all the way down the bore.
  • Hop up rubbers are okay, but they mounds are large (like Guarder) and are extremely greasy out of the box.


  • Piston head needs new O-ring that properly fits and seals with cylinder (#14 is recommended) and doesn’t come with bearings (Guarder Polycarbonate includes both of these features and is what I’ve always thrown into clients KA M4’s for upgrades).
  • Some of the KA M4’s that Julian got into Airsoft Gear had feeding issues out of the box – some KA M4 air nozzles are off spec, we generally fixed this with getting a M16 VN/M733 Guarder Air Nozzle from ASG (they’re a little shorter)… though both of mine fed fine out of the box.
  • I burnt out the stock switch assembly in about 3 game days of running 11.1v 20c LiPo hard on semi (2000-3000 rounds a day) – I’m thinking that I should go to VFC when my current parts-bin-scrounged SHS switch contacts burn out.
  • One of mine broke anti-reversal latch spring – causing anti-reversal latch not to catch (the latch itself seems to hold up fine, though it the axle is slightly smaller than the hole in the gearbox shell, causing it to pop out prematurely when attempting to put the gearbox shells back together… Guarder anti-reversal latch fits a tiny bit better and includes the spring).
  • Motor is standard type – runs pretty fast in full auto when sub-360/380 fps, but gets hot really fast on any spring above 400.
  • Selector has a tendency to want to catch on the outside of a bearing on the gearbox if the gearbox is not secured in a particular order (takes some fiddling to get everything working right upon re-seating of the gearbox into the lower receiver sometimes).
  • Cut-off lever is made from a less-than-ideal metal and tends to wear out, causing full auto or burst fire when semi-auto is selected.

Other notes:

  • Cylinder has 3/4 port – I don’t know if this is a pro or a con. I still don’t get it; I’m still using full cylinders on both my KA M4’s, even for a 300mm barrel length with no adverse effects so far as I can tell.
  • These guns do go for cheap – they’ve been going for $240-280 in Canada recently, if I am remembering correctly. With a little bit of TLC they work great.

Edit 04/04/2013 –

The following is another PM I sent to a different player in regards to what should be upgraded in a King Arms Colt M4A1:

Basics of King Arms M4 AEG upgrading to get it game-ready:

Piston head O-ring
Metal spring guide (ideally steel and with bearing)
Clean hop up rubber (change it if you don’t have the split mound bucking that looks like this: -v- )
May require spring downgrade (depending on what spring was installed in there in order to legally import into Canada)

If and when you want to do more:

Torque motor
Piston (stock one is okay but doesn’t last very long when shooting lots of full auto on 11.1V and at 350+ FPS)
Hop up nub (harder the better, or can go with some sort of fancy one)
Cut-off lever (stock one will wear out pretty quick, disabling your semi-auto)
Anti-reversal latch (specifically the spring – all 3 KA-branded ARL springs I’ve used broke in exactly the same spot)
Air nozzle (stock one is okay, but doesn’t seal perfectly and might crack after lots of use)
Switch assembly (stock one will eventually burn out on high voltage/discharge rate LiPo batteries)

For even more consistency:

Bearing piston head
Double o-ring cylinder head
Tightbore barrel (stock barrel is very consistent, results are pretty similar to stock TM barrels from what I’ve seen)

Optional, if you require it:

Reroute wiring to rear (if you so desire, will require splicing wires or soldering on whole new wires)
Selector lever (can be finicky to reinstall gearbox after taking it out to work on it – replacing it helps)



3 responses to “Review Request: King Arms M4

  • plan4

    I use the King Arms MOE M4 as my primary, running an 11.1v 20c lipo and .28 bbs.

    The gun is surprisingly on par with my friend’s KWA SR10 in terms of accuracy out of the box. I upgraded the barrel to a Madbull 6.01 tightbore, but the stock barrel proved to be more accurate, despite being listed as a 6.08 and I reverted to it. This was determined by direct, repeatable, head to head testing.

    With an 11.1v lipo, the ROF is quite high, high enough that I plan to use a Bravo AMU3 to reduce the ROF.

    The gun is heavier than my G&P Sentry, but lighter than a KWA.

    After approx. 20,000 rnds, I stripped the piston.

    Internals are a mixed bag. Some parts are great, others mediocre. Some observations:

    The front wiring is fine, but there are no bullet connectors, and not enough play in the wiring to allow the upper and lower receivers to separate without cutting the wires. I added bullet connectors.

    My cylinder was not ported.

    My nozzle has no o-ring.

    The stock piston and piston head are of average quality and are good candidates to be replaced.

    The 7mm bearings mean that when shimmed properly, the gears spin SCARY smoothly. With only shimmed gears installed and no play, slightly tilting my mechbox causes the sector gear to spin.

    There are no bearings on the piston head, nor on the spring guide, which is plastic. The mainspring was substantially distorted as a result.

    The cylinder head is your standard plastic/metal, single o-ring unit. Either a teflon wrap or replacement is necessary to improve airseal.

    After 20k rnds, using the 11.1v 20c lipo, I observed almost no wear on the trigger contacts. I now have a mosfet wired in, so no worries down the road.

    • overhoppers

      First off, thanks for voicing your experience with your KA MOE M4!

      I do have two things to further comment on, though:

      King Arms stock 6.08 barrel is extremely consistent in bore, much more so than any Madbull offering (6.01 or 6.03 – note that from many others’ scientific-ish testing bore tightness doesn’t necessarily dictate accuracy, but rather bore consistency does). So yes, exactly – the stock King Arms barrel is actually very good for the cost of the gun.

      In my opinion, the stock wiring harness is best dealt with (if in a front wired configuration) with a Tamiya pin pusher and leaving the stock mini-Tamiya plugs on there. All it then takes is disconnecting the Tamiya connector from the two leads and the upper receiver can then be easily removed from the lower. In my experience on and off the field, I’ve seen way too many bullet connectors come loose for no reason at all while gaming an AEG outfitted with these.


  • Tristan "Goggles" Wong

    I just bought a King Arms M4 (leaving it front wired because I intend to Magpulize it to make it as light as possible) because of these two reviews… thanks guys. Hopefully the overhaul I plan to put the gun through on Friday will be worth it. Fingers crossed.

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