Home ArniesAirsoftArn's Blog MadBull M4 metal receivers – first impressions

MadBull M4 metal receivers – first impressions

by Arnie

Madbull have a new body out and I’ve had a chance to try out the first Madbull body to roll out of the factory.

First things first a big thank you goes out to Simon at Madbull for making this article possible.


In the box for the M4 body you’ll find the new Madbull hop-up unit complete with hop-up rubbers, various bits for the receiver in a plastic bag and the body itself. Madbull highlight from the outset that the receiver requires parts and professional skills to install. Obviously you’ll need donor parts and items such as charging handles, select levers etc to complete the build. All the parts are included in the box that are specific to this receiver such as the body pins, gearbox pin and split pins that secure various latches and pieces in place. You’ll also find inside instructions for the hopup and the receiver itself.

The donor rifle for the build in this case has been a new Marui M733 so do keep in mind that my comments relate to Marui compatibility.


Lets face it -if you’re going to fit a M4 metal body you are either doing this from scratch and know what you are doing, or have a TM clone which you wish to upgrade and have parts on the table ready to go. My advice is that if this is the first body you’ve ever installed just take your time and don’t force anything (put that hammer back in the box…).

The Madbull body is a two piece metal shell which is held together at the back by two metal pins that point backwards from the upper receiver and then locate themselves above the gas recoil area of the lower receiver.

When you take the parts out of the box you’ll note that the top and bottom half glide together and fit to a tight tolerance. Obviously if you get to the point of putting things together and it’s all tight you know something is wrong somewhere.

IMG_8445You’ll also notice once the receiver is out of the box that not only is there a blank plate to fit for the other side of the fire select switch, but there are also mock rivet pin ends to fit. You will need super/crazy glue for this job. If you’re out buying some glue for this, grab yourself some red 243 Loctite “lock’n seal” glue as well for the screws.

The receiver has full PWS marking on it but according to promotional material various licensed versions will be available shortly and you can also order your own custom markings.

IMG_8444In my case I’m building a PWS Diablo (see details on the real PWS here) with the custom Madbull front end. Prior to actual  assembly I tried all the parts for fit and found that the barrel is a tight fit to the front receiver. Now this isn’t a bad thing as you want the front end to be like that. After some careful assembly and tight screwing down of the barrel nut and barrel set I found the front end to be a beautiful fit.

(rail covers are MaxTactical rubber bamboo rail covers)

Read the instructions… Next time I’ll remember to fit the dust cover and cover pivot before screwing the front end down. Remember those instructions that came with the receiver? Well if you read them (I did the standard male thing of ignoring them) they will tell you to do this before fitting front end.


Learning my lesson I have carefully moved on and set to fitting the gearbox into the lower receiver. I took a bit of time of this as I have wired my gearbox to operate with either a front or rear battery. I have a wire to feed either side of the gearbox, starting from the magazine well. The front wires head past the hop-up (one either side) and then into the barrel passing through a machined hole above the front receiver pin. One of each of the rear wires heads along each side of the gearbox and then to the back and out the stock pipe. There is no hole cut in this receiver to help this, so my install was a little tight. I just took my time, hid my hammer in the cupboard and spent an hour or so getting it all as I wanted it.

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The hop-up supplied is the Madbull Ultimate Hopup M4/AR which literally takes seconds to fit to an M4 barrel. Just make sure that you have some silicon spray and/or oil to hand to help in the assembly.

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There’s not much point saying how it all goes together, as a video says a thousand words:

If you fancy fitting the tracer board it is an optional part and as you can see from the hop-up photo dead easy to attach to the side of the hop unit.


So far so good in this build I have yet to fit the pin/rivet covers on the receiver and I’ve not used a punch to make all the pins flush, but I’m not finished yet.


Impressions so far? The receiver is well designed and the lines and fit are obviously manufactured to tight tolerances making a good looking piece of kit. You should be able to see in the photos how close the fit is at the front end. Even without an upper RIS mount across the body and front end this replica is already rigid and strong. No sign of any wobble or spinning front grips!

You will have to bear in mind that like any part the hop-up and receiver are made to be as universal as possible, however with a hundred different copies of duplicates, various rip-offs and clones out there the fit of the hop-up unit will vary. This is why a certain level of expertise is recommended when building a new receiver.

Like I said I had no issue using stock TM guts (reliable and bomb proof) and I suspect that the other main manufacturers will also have no issues.  Quite simply this looks like another hit from Madbull, who very shortly will be selling all the parts needed to complete a custom rifle from scratch!

More details will be coming as I complete this project.

I have no idea on RRP at this time but knowing Madbull it’ll be very reasonable.

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