Foreword..
01/10/2007 - Developping my blasters with sound and light Fx, I quickly stepped on airsoft replicas. Interesting cultural wave born in Japan at the beginning of the 80's, when the possession of firearms was illegal, consequence of the war with the USA, airsoft gave birth to incredibly accurate replicas of weapons, often in metal, shooting non dangerous (almost) 6mm plastic balls. My first airsoft was a slightly overscaled replica of a mauser broomhandle I wanted to convert into a SW blaster. Much cheaper than a Denix and far far more room inside to put electronic boards, LEDs, battery and speaker.
Then, browsing over online airsoft shops, I saw a kit proposing to build the replica of the assault rifle used by the USMC (U.S. Marine Corps) in Alien 2 (Aliens), the M41-A or Pulse Rifle.
I therefore started some intense research to better understand the origin of this prop, how it was built in 1986. A website is now gathering various kind of info about this fictive weapon : m41a.com. (I've been selected PR of the month for November 2007 !!!)

Building your own M41-A isn't cheap. It might even cost you an arm and a leg if you follow the royal way that is proposed most of the time.
Just like everyone, I'm on a budget, I therefore decided to calm it down and not to endanger my wallet.

Here are the key points I wanted to achieve :
► reduced budget
► working airsoft version (I don't go skirmishing, but it's nice to shoot a few BB on a paper target in the back of the garden, once a year)
► usable in costume conventions without any risk
► be able to play sounds like in the movies
► with a working bullet counter, but without the need to shoot BBs

The result in video





History of the weapon
This fictive weapon was supposed to be manufactured by the ARMAT company. As a prop, it has base parts made of real firearms and it's been designed by english armourers / props makers for the movie. The main part is a WWII sub machine gun, the Thompson M1A1.


The grenade launcher part of the M41-A is based on a Remington 870 shotgun and a SPAS12 cage. The whole thing was then covered with 2 custom shaped aluminium halves. The special Fx department then added the famous "95 bullet counter" on the right side of the rifle.

My PR version
I started my project in early October 2007. 2 weeks before, I ordered the Thompson and the PR kit on the Web.
Most of the fans starting building a M41-A use the airsoft replica from the excellent manufacturer Tokyo Marui, spending about 350 €. This is maybe useful for someone practicing outdoors airsoft guerilla, but in my case, I prefered to buy a cheap chinese version. Cost : 160 €, s&h + customs taxes included. It's an electric airsoft (AEG), very well made, metal gear box and it shoots straight as hell. The Marui might be more robust, but with my newbie eyes, I think I won't see the difference.


Then comes the kit to "wrap" the Thompson. Several solutions exist, you will find here and there some resin kits or vacuum-formed kits, generally designed to make non functionnal PRs (non airsoft I mean) but not always.
For the airsoft version, you don't have a lot of solutions. Sure, you can build up the shroud with styrene sheets, but it might not be as strong as needed to handle the weight of the Thompson. The airsoft manufacturer G&P made a very good kit a few years ago, but it seems to be out of stock everywhere. I found two sources for this kit, which seems to be manufactured by Merit International, I have no idea if the designed has been forked, if this is the same G&P kit sold as a different brand, or simply an old stock of G&P kit.
You can buy the kit from either Monstrer in Motion, or Airsoft Extreme, where I got mine, it's the cheapest I've found ($395). Stay away from the website called M41A model kit, the owner is just not responding to email.
Add the shipping cost by UPS, about $100. Hopefully, the USD is really low compared to € !
The kit comes in a big box and 2 levels of polystyrene very well protecting the parts.

Add then the short Thompson magazine. The original 40 BB magazine is far too long  to fit the kit. Marui makes this magazine, it's the high capacity 190 rounds model. I bought mine at airsoft extreme to save on shipping and on research time ($38).

I knew absolutly nothing about airsoft when I started this project, and I had in particular no idea is parts from a manufacturer will fit another etc. No real certainties that the Marui magazine will fit the Chinese M1A1, or if the kit will match it. I took the bet, and it all worked ! Just make sure to oil a bit the magazine for easy removal.

Absolute fans might say that the kit is too "plastic looking", and certain parts are not screen accurate. Well, maybe. But if you want to make something just like the original, like TK-560 did, you need to buy original military parts, and you might have to live in the USA, since it's definitly not permited in Europe. I think that the kit is really well done. I thought I'll receive a simple plastic thing, but everything is fitting great. The plastic is fibered ABS (probably with glass fiber inside, you can hear it when you thread a hole) and structure aluminium 6063. Only the instruction manual is not good enough, just poor quality pictures with lack of meaning full anotations. I'm going to make a PDF version to improve that.



Assembling the kit
The kit comes with a CD-ROM filled with building pictures, maybe made by airsoft extreme themselves. Some pics are blurred, but it's mostly the lack of annotations that makes the building steps sometimes boring.
PDF assembling manual

Dissassembling the Thompson is easy... when unlike me the headless allen screw on the top part of the barrel does not break (step 8 of the disassembling process instructions). I had to cut the top of the barrel until I've remove all the screw. This 10 minutes after starting working on the project : very stressing. The Thompson is mostly made of zamac alloy, with a lot of lead, and it heats up very quickly, making the cutting disc sliding on the metal rather than cuttin it. But it went ok without harming at all the internal airsoft 6 mm barrel.
As for the rest, it's mostly about removing screws here and there and the wood plastic immitation parts.


Once the Thomson is naked, we can start assembling the kit.


My nightmare in that kit has been the front rotating hatch, supposed to give access to the battery. It's made out of a really hard spring. Despite a lot of effort to make the thing staying in place, it finally broke the assembly hole on the side of the SPAS cage.
Once I've repaired, I decided to modify the parts to get a screwing system, very practicle too.




















Power & battery
I've been unable to find real and good information about the battery to use for the PR. Not even explained in the assembling pictures. The original battery pack of the M1A1 goes in the stock. It's made of seven 5/4 SubC 1200 mAh nicad elements for a total voltage of 8.4V. Since the stock is replaced by a retractable model, there is no more room for the battery : it has to be stored in the grenade launcher.
For the pictures on the Web, and in the assembly instructions, I figured out it was a stick battery made of seven 2/3A cells. I also needed some mini-airsoft pack to replace the stock one of my P90, it's using the same cells but with a 3+3+1 layout. I bought everything from the same shop,  all-battery. But what a surprise when I tried to use the stick battery : it just shot once or twice and the battery was dead. I though it was the gear box, or the wiring, I tried many debug test, and ended with the conclusion the stick battery was not strong enough.

However, the 3+3+1 pack, same capacity was working like a charm. Same cell format, same voltage, same technologie (ni-mh) and same capacity for the 2 packs : I couldn't figure out. Then I went back to the description of the items at all-battery.com and discovered that the 3+3+1 mini-airsoft pack was using special cell accepting 10C as discharge current, hence up to 14 amp, while the stick battery was using normal cells. I therefore unwrapped the 3+3+1 pack, with a lot of precautions to avoid breaking the tabs connecting the cells together, and I rewrapped the pack as a stick using heatshrinking tube.
Conclusion is : for your PR, you need a seven 2/3A 1400 mAh (or more) ni-mh cell pack that accepts high discharge current.

M40 Grenade
The kit is provided with a mecanism simulating the reloading process of the grenade launcher. When sliding the pump handguard, an aluminium cylinder reveals a plastic shotgun cartridge glued in the body of the SPAS cage. Except that this red thing is not at all what is used in the movies. I therefore machined on my lathe a simil replica of the M40, not totally accurate since I don't have the required equipment to make the grooves on the center of the grenade. The cap comes from a big pen, like the one you use on a paper board during brainstorming sessions. Not perfect, but that's a start.


Customization
Building "just" the airsoft version of the M41A will keep you busy for about 2 big afternoons, with the required parts and tools. On my side, I did not want to stay there.
First point, the bullet counter from the kit is made to work only with airsoft BBs. The system is really well made and designed, it's rare to find such a thing in a DIY kit. The electronic of the bullet counter is a little MCU handling the magazine detection (counter set to 00 when you remove it) and the decrementation of the remaining ammo using an optical detector made of an IR transmitter and receiver seing each other on each side of the barrel. When the BB is thrown, it cuts the IR beam for a short time and creates an electrical edge, and an ammo is decremented. The problem is : when you want to use it during a costumed convention, you can't get the ammo counter working since you're not going to shoot BBs everywhere in a public area.
Second problem : once the 95 ammos shot, the counter rolls over from 00 to 95 again. This to make the thing more practicle and also because the Marui short magazine has 190 rounds. I like to have an accurate simulation, so this is not really suiting me.
Third point : the kit does not have the grenade launcher trigger, I want to add this detail.
And finally, last point : the airsoft M41A does not play any sound. I'd like to have the following sound Fx :
► boot sound : the sound when you start the electronic module (including the bullet counter), just like on blaster core or in my illuminated sabers. Fabric/leather sounds, electronic beeps, cric-crac mechanical sounds, or whatever...
► the specific sound of the PR. Plurality of sounds, randomly triggered, or in sequence.
► shotgun / grenade launcher cycling sounds (2 different sounds for up and down)
► grenade triggering + explosion
► magazine removal / insertion sounds (2 differents sounds)
► empty weapon

M41A version of Blaster Core
My Blaster Core module perfect fits the need of the desired interaction of my PR, since it already plays sounds. We just need some modifications in the code to handle the I/Os differently.
We will remove :
► the 6 animated LEDs of the energy bargraph
► the handling of the graphic display (well, maybe I'll leave the code)
► the 6 progression LED of the blaster ray

We keep :
► the aux button, used for detecting the main magazine
► the single shoot / full auto selector
► the main trigger
► the main flash circuit, to get a nice bright flash each time you shoot
► the aux flash circuit, triggered by the grenade launcher

We also need to modify the electronic circuit of the bullet counter provided with the kit so that it becomes a slave of our Blaster Core. First, I've ripped away the optical BB detector from the barrel holder. Blaster Core will output a short pulse each time a shot is produced and this will decrement the counter. We just need to adjust the shooting rate of Blaster Core when in full auto so that it matches as close as possible the airsoft rate.
The reset of the counter is disconnected from the limit switch that detects the magazine. The wire goes to Blaster Core that drives that pin. This way, when powering up the PR, the counter starts with 00 whenever the magazine is there, then only when the boot sound has been played AND the magazine is in place, the counter moves to 95.
Once the internal count of the remaining ammo handled by Blaster Core reaches zero, the 2 digit counter reset pin is set and 00 is diplayed. If the shooting rate is carefully tuned, it will match the exact number of thrown BBs.
Once empty, the magazine has to be remove and inserted back to reset the counter to 95.

Getting the shooting electrical info from the airsoft trigger
The airsoft part is electrically separated from the electronics of the ammo counter. Fortunatly, cause the airsoft motor is using like 15 amps when started and we need to maintain this electrical isolation. To get the info of the trigger being pulled, we need to connect on the power supply of the gearbox motor. Neophy has proposed to use a little reed relay for that purpose, however, I've prefered to use an opto-isolator for the following reasons :
► a relay, especially with a roll-off diode can be a bad load in parallel with the airsoft motor
► some models use quite some current
► mechanical stress and bad electrical contacts on the relay might lead to double triggering on the electronics of Blaster Core

I've therefore wired a classic TIL111 opto-isolator on the power supply of the motor, since we don't need a particularly quick part, due to the slow rate of the airsoft. To install the wires, you'll need to remove the handle and eventually the whole gearbox from the M1A1 receiver to get the wires passing thru. The wires to the TIL111 were simply inserted in the clips attached to the motors.
A 220 ohm resistor in serie with the LED of the TIL111 will give between 20 and 40 mA whatever the battery state is, and it's far enough what is needed to saturate the output transitor even if it's not a darlington. A pull-up resistor of 47k produces the idle level on the output of the opto-isolator and completes the open-collector circuit. We then get a circuit providing a high level (3.3v) when the rifle is in idle state and it goes down to 0v when the airsoft motor is activated.
Note : in the case of a non-airsoft use, a simple limit switch will achieve the same function, just like in my blasters.

[opto isolator schematic]

The falling edge from the opto-isolator is really straight, but the rising edge, when the motor is stopped is full of ripple and is not really beautifull (intertia of the motor, air finishing to be blown, coil being emptied etc). To make sure it will not trigger several ammo on my board, I've filtered the rising edge with a 1µF tantallum capacitor in parallel of the pull-up resistor. It's enough to clean the ripple without increasing too much the rising time of the edge. We end with a pretty nice waveform as seen on the following screenshot :

The opto-isolator was "spider wired" (I don't know the expression in english) : all parts are directly soldered on a tulipe IC holder, glued then wrapped into heat-shrinking tube. It fits perfectly the available room close to the retractable stock mechanism and the output wires from the opto-isolator return to the SPAS cage where the electronics is stored, following the same path as the battery wires going to the airsoft motor.


15/11/2007 - I feel dumb I haven't thought about that before... If you wish to keep the system as is but with a possible desactivation of the airsoft motor, it's very easy. As a matter of fact, it's not great to use the airsoft motor without BBs... and the noise covers the sound Fx from my board. Just remove the handle (one single allen screw) and one of the electric connection of the motor. Just make sure that the optocoupler wire that is wired there remain connected to the plug. Isolate the plug with tape so that it can't make contact with anything inside the handle, and put the handle back in place. Here you are, the motor is shutdown and you can fully enjoy the sound Fx !

The M40 Grenade Launcher
To get the proper interaction with the shotgun pum & handguard mechanism, it's pretty simple : we just need a limit switch to be placed where the inner aluminium tube that hide and reveal the grenade in the SPAS cage ends its course.
I used one of the holes meant to attach the SPAS cage to the shroud. I just threaded the hole in M3, mounted the limit switch on that with some blue thread-locking liquid.
The kit does not come with a grenade launcher trigger, that's really too bad. I'll have to make one in a thick aluminium sheet and attach a second limit switch to trigger the associated sound. This will be part of the future improvements, for now, I prefer to stabilize the whole thing and finish the code of Blaster Core in this special Pulse Rifle version.

Light Fx
Blaster Core can handle 2 circuits that flashes when either the main or the auxiliary weapon is used. For the main flash, the device has an on-board power extender that can drive a load up to 1.5A. Perfect for a luxeon LED for instance. The aux flash circuit is just an ouput of the microcontroller and ouputs 3.3V / 25 mA. However, using an external power extender board (soon to be released at Plecter Labs to go with Blaster Core) it can also drive a powerfull LED.
Once I've remove the optical circuit of the bullet counter, I drilled a 5mm hole just under the output of the barrel and I placed there a 5mm high brightness white LED, perfect as the main flash of the PR.


The front cap/cover of the SPAS12 was drilled right on the fake allen screw and I inserted a chromed bezel for another 5mm white LED. The bezel will be sanded and painted in military grey later. I wished I could insert the LED inside the hole of the shotgun barrel, but this is unfortunatly impossible now I've modified the cap system.


Magazine and Bottom Cap
The cap that goes under the main magazine is a real problem with the kit I've used. Once the magazine is in place, the cap is supposed to clip on the side of the shroud, unfortunatly, it barely stays in place, and will be pleased to fall off as soon as you start walking around with the PR.
Second point : this topology is not screen accurate. The cap is supposed to be attached to the magazine. I therefore need to find a way to attach the magazine to the cap. Some resin ready-made cap exist, but I like the idea to stay on my budget and use as much as possible what came with the kit.
Additionnal problem : the Marui magazine is spring animated. The little roll-on mechanism to tighten the inner spring is off course located... under the magazine. Impossible to simply glue / screw the magazine to the bottom cap.
The solution ? a little motor placed between the magazine and the cap. I need to find a powering solution, though. Ideally using the inside battery of the PR with a little connector and switch.
If this works, it will be possible to remove the magazine and the cap in one block, fill it with BBs, put it back in place and power the little motor to load the spring.
Just like the grenade launcher trigger, I keep that for later, as an upgrade of my project, for now I want to stabilize the electronics.
15/11/2007 - I'm going to modify the existing bottom cap. The idea is to add a holder inside that will be attached to the cap and will fit the magazine, to remove the whole thing at once, to reload the PR. A window will be milled under the cap to access the windup mechanism.

M40 Grenade Launcher Mechanism & Trigger
It did not take me long to install all that. For the trigger, I salvaged a really cheap plastic airsoft gun. No remorse, here we get a nice trigger


The trigger was attached to the SPAS cage with a threaded spacer glued with cyanoacryalyte glue then epoxied. Same for the limit-switch I've used to make the electrical trigger, and it provides also the spring effect that sends the trigger back in place. The whole thing has been then epoxied with bi-component putty and it's now hard as a rock.
To gain some room for the trigger and my electronic board, the fuse as been moved close to the 9V battery compartment, using some additionnal wire length (silicone coated wire, to better handle the current taken by the airsoft mechanism). The fuse protection plastic case has been replaced with heat-shrinking tube (non retracted).

Cheating a little bit...
Let's be honnest, an accurate simulation is great but... removing the magazine to reload the counter to 95 can be boring with that heavy weapon.
To make demonstrating and playing around "easier", I've added a little secret button to do as if I'm removing the magazine. It's a limit switch (to get a normally close contact) wired in serie with the original limit switch that detects the magazine. When pressing the button I cut the signal and do as if the magazine has been removed. I've installed that on one of the fake screws on the loading hatch of the shotgun. It remains this way really accessible when naturally holding the rifle. I've used a silicone button salvage from an old gamepad to transmit the pressure to the inner switch, epoxied on the plastic piece.


My M41A (almost) finished - 15/10/2007

I think I'll use the sling provided with the M1A1. With just a few modifications, it will look good enough. However, the PR kit I got has nothing to attach the sling, so I'll have to modify the stock and the barrel holder so that I can attach the clips of the sling.
For the front clip, it's supposed to be tied to the barrel holder, near the output. Accurate metal kits exist, usually supposed to be used with a real SPAS cage. On my kit, I have a plastic replica of the SPAS cage, which is not so rigid. I've been afraid that holding this heavy weapon from the barrel holder would harm the inner airsoft barrel after some time. I therefore prefered to use a screw on the left side of the SPAS cage to attach a little D ring I got extra with a cheap P-90 AEG. Not screen accurate (but is my PR screen accurate at that stage ? no, for sure !). Simple, cheap and efficient, though.
For the stock clip, I had to cope with the fact I have 2 big allen screws attaching the rear part to the retractable rail. I used a U shaped nail that I scratched with a dremmel cutting disc to create some "harpoon" teeth. The 2 holes were threaded, also. This way, it creates obstacles in the holes and get stuck with resin (technique of the "prisonner", in French).

21/10/2007 - While searching for info about the PR, I landed several times on the must-be-seen website of Phil Steinschneider. A section is presenting the work of Derrick Baena on his M41A, introduce the incredible Pulse Rifle Manual he wrote, and the fabulous case he made for his replica. Since I'm always looking for some room at home, I needed a way store my PR anyway, so a USCMC storage case was definitly the way to go !

The foam that comes with the case is low density polyurethane foam, just like the one used for the flight-cases. I dislike the aspect, not very esthetic, it easily create plushes and it's horrible to cut (you need a thermo knife or a roast-beef electric knife to do the proper job). And it's not strong enough to hold in place a heavy weapon such as the M41A.

From the pictures, I guessed that Baena has used polyethylene foam. Here again, I had to do some research about the different foams and their use. And finding the cheap black foam of the right kind in France is simply a nightmare. Here is what is commonly proposed on the market :

- Plastazote foam. It's a trade mark / brand. PE foam, pretty dense, really smooth and waterproof (the cells are closed), not sponge behavior. It's also what is used for creative work, to build little foam buddys, costumes, flexible armors. It's also used to build surf body boards, and various kind of floating materials used in swimming pools. Hard to find in big thickness / long + large boards and  very expensive. Filling my case with it would have cost me like 100 euros. Alternate solution : 15 mm thick tiles used to protect the floor of sport rooms from the weight of the machines. Found at Decathlon, 15 € for 4 black "puzzle clip" tiles (60x60 cm)

- American Foam : (called like that in France, or Ethafoam or E220). Made of polyethylene. Used for kayak boats or for building river rafting mini boats. Cheaper than the Plastazote. About 100 euros pour 275 x 60 x 10 cm (4 times what I need for my case).

- "Simple" Polyethylene foam : rough cells, closed or open depending on the use. You find it in everyday life, to protect computer hardware, hifi appliance, fitness / gym mats etc. That's what I've chosen for my case.
 

Like we are often used to, in France, we need to buy in quantities for interesting materials and the project suddenly becomes way to expensive, except if you are doing a team work. I therefore looked for some parts / objects I could salvage or adapt to get some foam to put in my case. Camping or fitness floor mats are a well known and are classic methods to get some soft foam, for instance to make fake mediaval weapons. Main problem : black is kind of non existant, and you often find only green or blue (fitness) or brown / grey (camping). The camping version is thicker (1 cm), but you pay for the underneath metalized layer (supposed to improve the body isolation from the ground).

I went for 6 red (!) mats at Decathlon. Original color, going well with black and grey. Dimensions : 150 x 60 x 0.7 cm. 3.5 € each. 21 € total, I'm staying on my cheap line. I'm going to glue them with neoprene glue to get the right thickness. Then I'll cut the shape of the PR & its accessories.
29/10/2007 - I've changed of strategy for the foam : I finally found a foam supplier close to my place. I went back to polyurethane foam since I found some much more dense than the one I use to get. This time it's some 43Kg/m³. I got the foam custom cut at the shop for free. I have one of 2.5cm and the other of 5cm thick. The 2.5cm will lay on the bottom of the case, and I'll cut the shapes and patterns thru the 5cm one, with a sharp meat knife, as the selled adviced me. Cost : 25 euros, only 4 euros more than the gym mats. I got refunded for them, without any problem !

Looking for a case on ebay, I found by accident a seller proposing somes stickers similar (replicas ?) of the ones seen in Derrick Baena's PR case. Perfectly acceptable price and easy communication with the seller. It's there.

05/11/2007 - Stickers Update. I've received the stickers this morning. While the big white stickers are excellent quality, the set of 6 little stickers (PR nomenclature, serial number, package contents etc) is highly pixelized like it's been printed in 72 dpi instead of 600. Some are a mixture between bitmap graphics and vector graphics, and you even sometimes see the jpeg compression artefacts in the non uniform background. I'll unfortunatly have to make some of those myself.

05/12/2007 - PR case update. I've finished my stickers. It took like 8-10 hours.  The case is almost done now, only the six M40 grenades are missing.

 

►28/09/2008, Update : After moving to a new  house and 4 months working in it, I'm back to props work and I can (almost) finish the PR. The magazine cap was still not attached to the magazine itself. Thanks to mnoble2 on aliens legacy, his wonderful system fits a stock G&P magazine cap and the Marui HiCap magazine.
Also the detail of the wiring under the handle : to desactivate the airsoft motor (and save the mechanishm from dry firing) it's as simple as removing the red wire clip. Notice the thin wire that stays connected to the clip, that where the fire info is taken. Pulling the trigger sends voltage there but it only goes to my board and not to the motor. A little piece of tape is enough to isolate the clip from the motor connection.


I'm also now a lucky owner of D. Baenna's PR user's manual, a wonderful piece of art. Like all the manuals he's made, it's custom printed with my name and rank on it.
The M40 grenades are still missing, but I have to wait until my workshop and lathe are functionnal again.


►Update of 20/05/2010 :
It's been a while since I've received my M40 replicas (m41a.com), I finally updated the case to house them. Labels have also been taped on plasticard and look much nicer. Next time I need plates like that I'll probably get them laser cut.