Accessory Review: HAO Geissele REBCG (Ecoline) For Marui ZET/MWS GBBR

I’ve (for the most part) avoided buying internal “upgrades” for my ZET/MWS rifles until recently, mostly because the primary business that’s offering parts for the platform, Angry Gun, appears to think that a photo constitutes a technical drawing and making something “sort of good enough but not really” is actually acceptable.

From Ambidextrous selectors and mag releases that fail to function as intended, to Mk16 rails with QD points that simply don’t work! It makes you wonder who’s actually testing their shit before it gets sent to retailers, or are they expecting their customers to do their testing for them?

Now that HAO have finally seen the light and are making a number of parts and indeed kits for Tokyo Marui’s GBBR line, those who refuse to throw money at poorly made Angry Gun parts (don’t even @ me) can actually furnish their builds with not only rails, but also a host of other parts and kits specifically designed for the ZET/MWS platform.

HAO’s competition destroying CAG 416 kit for Marui’s MWS/ZET platform.

One of the first parts HAO have released is their bolt carrier group. Available in either Heckler and Kock or Geissele REBCG “Tan G Stonks” (Don’t ask) flavours, to suit not only their H&K 416 kits (the number of which is expanding exponentially), and the REBCG to suit a number of M4 type builds.

Both REBCG and Heckler & Koch bolts, HAO taking the sensible step of not putting trademarks or indeed names on their website.

The REBCG or Reliability Enhanced Bolt Carrier Group is a Geissele manufactured component, designed to provide a more reliable alternative to many of the mass produced bolt carrier groups available on the market. Whilst it’s not part of the USSOCOM URG-I program or indeed a component of any other military rifle build, it wouldn’t be beyond the realms of possibility to see this being used by units or operators who have the flexibility and desire to use high grade, commercial off the shelf components.

So let’s look at HAO’s bolt carrier group.

Made predominantly from 6061 billet Aluminium with a steel weight on the rear, the main body of the bolt carrier is also hard anodised for longevity. This isn’t the same as the genuine article (which uses a chrome-nitride coating), but looking at the pictures on Geissele’s website, they’re actually not far off on the colour, which appears to be a silvery grey (with a subtle hint of tan).

The genuine Geissele REBCG, note the two exhaust vents on the flat face of the bolt carrier group.

Design wise, HAO haven’t changed much. The Marui compression parts such as the nozzle are a direct drop in fit, which is a massive relief, as it means any bolt carrier related upgrades and spares should also be a seamless installation.

The HAO REBCG with it’s donor Tokyo Marui bolt carrier.

One of the only noticeable differences, is with the rear roller being of a traditional bushing design that’s held via a flat blade machine screw, with a partial bar shank to enable the roller to spin effortlessly.

The roller, held captive by a flat blade machine screw.

Moving forward; the bolt carrier profile mimics that of the Marui it replaces, but with a few minor aesthetic differences. On the face of the carrier, the Marui is subtly chamfered, whereas the HAO is rather blunt. There’s also the markings on the right hand side of the bolt carrier, although these have been laser engraved not machined out as per the real thing.

The exhaust holes that a real REBCG has are strangely absent, but when you consider that the H&K 416 is piston driven, not DI, it makes a little more sense. The “G” that appears in its place is superfluous, the real REBCG lacking this detail. This is perhaps the biggest indicator that HAO designed and marketed the bolt carrier group primarily for their 416 kits, the REBCG being somewhat of an afterthought… Sorry HAO, but that’s how it looks.

The colour appears to change depending on its surroundings, here it looks rather silvery grey, much like the genuine REBCG. Note the “G” where usually you’d see a pair of exhaust vents.

A steel weight is bolted to the rear of the BCG, this is cross compatible with the original Marui weight if you should want to experiment. Other weights would be easy enough to make for those with access to a lathe, giving you a lot of options for bolt optimisation.

The rear of the BCGs, the weights being held on via a large bolt within the weight itself.

Sticking the bolt carrier on the scales revealed that it’s significantly lighter than the Marui bolt group, weighing in at a feather like 179 grams, compared to the Marui’s 220 grams. But does this have an impact on the performance? You bet it does!

Looks aside, I was interested to see if there was potentially any performance differences between a benchmark, stock Marui bolt carrier and this HAO version. Luckily, I own two ZET system builds and with both, I’ve maintained a completely stock bolt carrier set up. So taking both bolts onto the testing range, I was able to directly compare one with the other.

The test platform, my workhorse ZET/MWS system Mk16 “night fighter” GBBR.

After punching a few mags through the new bolt carrier to ensure it was properly bedded in, I started to look at the measurable results between stock and HAO bolt carrier groups.

Note the colour of the bolt compared to HAO’s DDC Mk16 rail and Unity Tactical’s FDE FAST Micro Mount.

Firstly, the rate of fire. Unfortunately the RoF function on my chronograph decided to play silly buggers and not record any data. I’ve tried a couple of times, but it appears that this isn’t a strong point of the Xcortech’s design. The increase in the rate of fire is certainly noticeable though, I’d estimate that there is maybe a 15/20% increase in the rate of fire using the HAO bolt, but I’d like to try and test it again when I get the chance. The ZET/MWS system isn’t a slouch, but with this bolt in place, it’s seriously rapid.

The muzzle velocity also appears to have taken a little unexpected increase, likely another side effect of the lightweight bolt carrier group. I recorded the results over a 30 shot string and using .3 ASG ammo, on the original bolt I found I’d get around 265fps which works out at just under 1 joule (bearing in mind it’s a cold time of year, I’d get just under 1.1 joules in the summer).

With the HAO bolt, I was getting an average of 280fps (1.09 joules) with the same ammo. This does mean that if you’ve got a build that’s close to the limit already, you might want to consider how you set up your build when installing this bolt as you might find yourself drifting a bit north of your site’s (or indeed country’s) legal limits.

Are there any negatives? Well, I did notice one or two lower FPS readings when conducting my tests. This could be due to a degree of bedding in that needs to take place, but it could also be indicative of the lighter weight bolt.

Lightweight bolts generally have a reputation for faster rates of fire, but at the expense of overall consistency. It’s something that long term might expose itself, but I’ve no reason to be concerned so far.

The bolt carrier to the rear, showing Marui’s faux lugs on the nozzle’s “bolt face”.

So did I notice any randomness in the shooting? No, the bolt change didn’t seem to affect range at all. Even when firing in full auto, I didn’t suffer from a single feeding or cycling malfunction.

Longevity, I’ve picked up on literally one area of wear on the bottom of the bolt carrier group. Bearing in mind that I’ve put around 1500 shots through the new BCG, I’d expect to see a little wear here and there, but HAO’s hard anodised finish appears to be keeping the BCG in good condition.

Note the single area of wear on the bottom edge of the BCG, this isn’t visible when the BCG is installed.

The big question as always, can I recommend it to a friend?

Yes… Yes I can. It’s well priced, works as intended and ultimately it looks fantastic. Would I suggest it as a potential upgrade? Probably not, the standard bolt carrier group works as well as it needs to, but if I had either a HAO 416 kit or a Mk16 rail (or just wanted something a little different), I’d certainly consider adding this to the basket. I bought this to add a touch of class to my otherwise vanilla ZET/MWS receiver, and it’s certainly made the bolt stand out! It’s not 100% correct, but given that this was designed primarily for the 416 kit, I’ll forgive the error.

A lighter, faster bolt carrier group paired with a shorter blowback system such as within HAO’s 416c kit could produce a pretty rapid rate of fire.

This item was purchased from HAO direct through their website: Like with all things HAO, they’re made in batches. So you might find that unless you time it right, they’re out of stock. You might also find that your REBCG is a slightly different shade, and that’s fine… I’m sure even Geissele wouldn’t guarantee the colour.

What else HAO decide to offer the MWS market remains to be seen, I’m personally hoping for a full M4 receiver set at the very least! So come on HAO, do what no one else dares attempt and make us a Vietnam era M16!

One thought on “Accessory Review: HAO Geissele REBCG (Ecoline) For Marui ZET/MWS GBBR

  1. Good shout, I’d be interested in your results over time as I’ve ordered one also.

    Don’t know if you’d be interested but I’ve uploaded a free spreadsheet to the mws thread of the airsoft forum to help keep track of peoples results of modifications Have a look and hopefully we can benchmark what’s good and what’s not in the after market parts world by standardising tests.

    Maybe make certain manufacturers change there ways.


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