Maruzen/TM Gas SPAS-12

[CatgutViolin]

This is less a project idea and more a summary of a project I've been working on, so if this isn't the right place please let me know.

 

Anyways, over the summer I got the idea to build a gas-powered SPAS-12 based on this video:

 

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I had no experience with either gas shotguns or the TM SPAS-12, and no way of procuring the latter as it is out of production, until a friend of mine with a broken and unsalvageable TM SPAS offered to donate it to the project. I also had the aid of shotgun guru who goes by the handle notrabies, who provided key assistance over the course of the project.

 

My first task was to prepare the SPAS-12 itself for accepting the components of a Maruzen M1100. This is what I had to work with.

 

 

The first step was to gut it entirely, following a takedown guide I found online, and remove all the internals, leaving just the shell. This worked just fine on the rear section of the gun, leaving me an empty receiver.

 

 

However, on the front part of the gun, there was a problem. You see, on the Marui SPAS, the magazine tube is fake as it loads from individual shells like any tri-shot. What this meant is that the magazine tube is not one piece, but two, and the point where they connect is both solid metal and a structurally important piece, as it secures the barrel and magazine tube to the front of the shroud.

 

The tech at Yuji Gun Works who produced the gun in the video at the top used what I believe is a rather lazy solution. In the first sequence, a total of five shells are loaded. In the following sequences, a total of six shells are fired each time. This indicates to me that what they did was use the standard-length magazine tube inside the shroud, and just left the faux tube extension from the Marui.

 

Well, I wanted the full 7+1 capacity of the M1100, so I had to come up with an alternate solution. The piece is too thick and poorly shaped to drill through, which left casting the piece as the only viable option. I used a two-part urethane rubber to create a mold, and then acrylic resin to cast the duplicate. Below is the original and the mold:

 

 

Once the duplicate had been successfully cast, I could fit and secure it to the SPAS body. Below is the gun, and above it the original (with gaps plugged with putty):

 

 

Although I had ordered an M1100 when I started the project, after this was a three-month hiatus as I tried to get RedWolf Airsoft to realize that their package had, indeed, been seized by US Customs, and issue me a refund. Eventually, after a long chain of emails, I managed to get a refund, minus significant Paypal fees which they forced me to eat.

 

At about this time, notrabies found a large deal on Maruzen shotguns on a classic airsoft forum, and we decided to split it. I received a full-length M1100 already rigged for external gas and a box of shells, as well as a Palmer's rig I had already ordered and a KTW folding stock (neither pictured). This M1100 had been upgraded as well, with the standard extractor replaced with a screw since extractor deformation is a major problem with the M1100, and a replacement lifter designed by Schaap and which you can see more of in his M1014 thread. Here is the gun itself:

 

 

I decided that I was going to fit the KTW grip to the Marui body, rather than fit the KTW stock to the Marui grip. In retrospect this was a bad decision that later required extensive structural reinforcement, but the modification to the trigger housing and receiver were straightforward and I soon had all the body parts completed:

 

http://i99.photobucket.com/albums/l296/catbarf/Airsoft/100_0205.jpg

 

Fitting the M1100 parts to the Marui body was extremely straightforward. Once disassembled, the receiver of the M1100 slides perfectly into the Marui receiver. Then the shroud is screwed on, and the magazine tube extension projected through the front of the shroud and screwed on. The grip is then screwed on, and with the grip pulling the receiver from one side, and the magazine extension pulling from the other, the body is held extremely rigid with zero flex. Attaching the grip, which was the KTW's, proved to be a little more difficult. In the end, I permanently affixed one half of the grip to the receiver, with the other half remaining removable so the internals could be pulled out. This solution is not optimal, and as I said if I were to do it again I would have modified the stock to fit the Marui grip instead. Here is the work in progress:

 

 

At this point, during testing, I noticed I was seeing issues with chambering a round. Occasionally the force of the magazine spring would propel the next shell straight back into the chamber, causing it to get wedged between the lifter and bolt, which would prevent the bolt from moving forward. This was a nasty jam that proved to be extremely annoying to clear, so I devised a simple solution. I superglued together some plastic card, then carved it into a small triangular prism, and affixed this piece to the bolt. It stops the shell from moving all the way back, and serves as a ramp to help feed the shell into the bolt. It turns out this problem was a consequence of Schaap's lifter design, and is not a problem with the stock lifter.

 

 

During this period of testing, I also ran into a number of other problems. The mechanism did not cycle properly and had some resistance to its motion, which turned out to be the shell being too tight on the shell door, easily fixed with some spacing. Another question remained how to make the pump functional. It turns out notrabies had done something very similar with his M1100s, giving them pumps from 870s, allowing them to use the pump to chamber a round or clear jams. This isn't exactly how the real SPAS works (the real one switches between pump and auto- on auto, the pump grip is locked in place), but it's very handy for airsoft purposes.

 

I took the metal pump linkage and hacked it shorter, leaving just the stub of one arm. I then attached two pieces of brass rod to this arm. When the pump is cycled, the brass rod pushes on the bolt and follows it into the receiver, opening the chamber.

 

Here's a decent shot of the gun:

 

 

At this point, it was ready for testing, using the Palmer's rig.

 

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It turns out that the source of the jamming was worse than I feared. The M1100 receiver was well-used when I got it and during testing had actually split on one side, giving just enough flex to allow the bolt to fail to engage the lifter mechanism properly and get stuck.

 

Through one of my online communities I managed to get an M1100 Revision and a bunch more shells cheaply. I swapped out the receivers, keeping the upgraded extractor and lifter, I also built at this time a shell catcher suited to the size of the SPAS, as shell catchers built for 870s and 1100s aren't anywhere big enough. I then took the gun to its first game, and it was a total disaster. The gun itself would jam up every ten or twenty shots, and all three of the other guns I brought failed spectacularly, which meant that I didn't have much in the way of successful combat performance for a first outing.

 

I decided to swap back to the original lifter, and found that it worked much more smoothly. A bit of tweaking to the gas system and hop-up also produced more consistent results and higher power, and I learned some better loads for the Maruzen shells to get the most out of each shot. I also took the opportunity to track down some gear, including shell pouches and a bandolier, as it quickly became apparent that being able to reload is the most critical part of using this sort of weapon.

 

It fared much better in its next appearance, at a zombie-themed game back in October. With most of the kinks ironed out it's become a significantly more effective gun, and I can't wait to use it at a close-range CQB game, as the field I usually play it is more field. Still, with hop-up, nine BBs punted at close to 300fps apiece have a good chance of hitting even distant targets, and it made the perfect gun for a member of a science team security detail:

 

 

Lastly, a few weeks ago I made a second video, this time live fire, to show the gun's operation when actually loaded.

 

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The M1100 is not cheap, the SPAS is not easy to find, the shells are not cheap, easy to find, OR convenient, and the external CO2 rig is more than a little awkward. But at the end, I think it's a badass and iconic weapon, with unique and more than reasonably effective performance. On my usual field it's a fun novelty, in a tighter CQB environment it could be absolutely lethal. If anyone is looking to do something similar, it's not as difficult as it looks, as most of the process was a matter of trial and error.

 

That about covers it. All comments and suggestions appreciated!

[Username_Here]

I love the working pump with the semi auto. It's surprising how easy it is to fit the M1100 junk into the springer bodies. I know when I made the M1014 everything fit pretty much right in.

[blingblip]

It's pretty epic no denying that, how many bb's fire per shell?

[WhiteHawksan]

that is pretty awesome, no doubt.

[Horsem4n]

so the maruzen m1100 receiver just fits right into the marui spas 12 body?

[CatgutViolin]

It's pretty epic no denying that, how many bb's fire per shell?

Up to ten. Eight .2s at 140PSI produces an undodgable cloud of flak good to about 100ft, while four or five .25s at the same pressure can usually hit a man-sized target at 150ft.

 

so the maruzen m1100 receiver just fits right into the marui spas 12 body?

Pretty much. The body is fairly roomy, and the M1100 receiver is pretty much a big brick with a barrel and mag tube. The main issue is making sure it's nice and secure, which means adding spacers where necessary and having multiple contact points with the body. With a spacer on top and the receiver secured via the magazine extension on the front and the grip on the back, it fits perfectly.

[Username_Here]

Up to ten. Eight .2s at 140PSI produces an undodgable cloud of flak good to about 100ft, while four or five .25s at the same pressure can usually hit a man-sized target at 150ft.

 

Pretty much. The body is fairly roomy, and the M1100 receiver is pretty much a big brick with a barrel and mag tube. The main issue is making sure it's nice and secure, which means adding spacers where necessary and having multiple contact points with the body. With a spacer on top and the receiver secured via the magazine extension on the front and the grip on the back, it fits perfectly.

 

One of the loads I liked doing when running external was to put a .30 in the back first and then 6-7 .20's on top. The .30 would generally hold a straight course while the .20's would just scatter out.

[CatgutViolin]

One of the loads I liked doing when running external was to put a .30 in the back first and then 6-7 .20's on top. The .30 would generally hold a straight course while the .20's would just scatter out.

That's an interesting idea and sounds like it would definitely make use of the hop-up, I'll be sure to give it a try next time I'm in possession of .3s, and I imagine a .25 probably wouldn't cut it.

[SpankyTheDolphin]

Awesome job, looks really good.

[Dagonet]

Outstanding dedicated labour of love.

 

Top show old chap, 

[aznriptide859]

I have to ask, how did you modify the KTW SPAS stock onto the TM SPAS? Was the modifications hard/permanent to the rest of the receiver?

 

Thanks

[Voxcaster]

Do you think you could get away with using a brass catcher on this, or would it cause it to stovepipe?

[CatgutViolin]

I have to ask, how did you modify the KTW SPAS stock onto the TM SPAS? Was the modifications hard/permanent to the rest of the receiver?

 

Thanks

 

I cut away on the inside of the KTW grip, and cut off part of the brackets on the TM receiver, and then attached the KTW grip (with stock) to the TM receiver. It was permanent and in retrospect a bad idea, because it significantly reduced the strength of the connection and I had to strongly reinforce it to get it sturdy enough that I could use the gun one-handed without it breaking. It would be much better to modify the TM grip to accept the locking mechanism from the KTW, and then just fit the KTW stock.

 

Do you think you could get away with using a brass catcher on this, or would it cause it to stovepipe?

 

I actually built one out of polycarb sheeting and a cut-up zippered mesh bag, I think I mentioned it in the second video but haven't taken any pictures yet. It holds about twenty shells, so it's good for a firefight. If I'm in a static position and need to put out more fire than that, I can unzip the bottom and then the shells drop out and it functions as a shell deflector, dropping them at my feet. I'll try to get some pictures up once I get back to school. I've only used the gun in-game with the shell catcher, because picking shells up is a pain even if I could find all of them when they're getting punted around the forest.

 

The extraction is actually weaker than it was before, I intend to open it up and play around with the extractor until I have it where I want it. When I initially set it up in the M1100, it was ejecting the shells a good 10ft. That kind of force beats on the shells, but ensures no risk of stovepiping during rapid fire or at lower pressure.