Tokyo Marui 5.1 HiCapa 3D-Printed Carbine Kit Build (Part 1)

Tokyo Marui 5.1 HiCapa 3D-Printed Carbine Kit Build (Part 1)

by Joker Tool Works


Disclaimer:
This is NOT a kit for real firearms. First, using this kit on an actual firearm would be a violation of the National Firearms Act, because you'd be converting a pistol into a Short-Barrel Rifle (SBR). Second, that would be INCREDIBLY dangerous, as no parts of this kit would survive under the recoil impact, gas expansion, and powder burn of a real firearm. This is EXCLUSIVELY for airsoft devices, which shoot a 6mm plastic BB at non-lethal velocities using air or high-pressure gas, such as CO2, "green gas" (silicon-impregnated propane), HFC134a (duster gas), and High-Pressure Air (HPA - compressed air).

0.1) Introduction

I have recently decided that, with my 3D printer, I want to create an original design for a carbine kit, specifically for Tokyo Marui (TM)-pattern 5.1 HiCapa airsoft pistols.

Objective: Create a carbine conversion kit for a TM 5.1 that will provide the following: a stable shouldering platform; a forward grip; second-magazine retention compartment; facilitate accessories top-of-receiver and under-the-barrel; able to sustain damage from regular use during airsoft matches.

1)  Conceptualization

The first step was designing a very simple version of the end product. This version, hereto referred to as V1 or Version 1, lacked several major details that would be visible on the final product. I wanted to capture design elements from series such as Mass Effect (specifically human SMGs and pistols) and Halo (like the SMG and Magnum pistol).

Initial Concept

After the basic concept of the kit was established, I needed to start modelling how the parts would lay and coexist with each other.

  

2) Version 1 and Prototype

V1 kit “finished”.

With the concept fleshed out, I didn’t like the stock design (see how it’s sitting lower on the guide rods than the inlets in the stock would suggest? I was having issues with how I wanted to place the stock). I initially had proposed a mixture of a gutter sight and a standard pistol sight arrangement. This would later be changed to a traditional gutter sight design. This design was used for two reasons: One, the primary use of this chassis is with a red dot or light-magnification pistol scope; Two, gutter sights are more than sufficient for airsoft purposes.

With the core elements all present, I decided it was time to print the first prototype of the primary receiver group to ensure proper fitment and operation.

Version 1 receiver group prototype. Printed in Hatchbox White PLA at 0.2mm resolution.

In the end, the tolerances against the slide were too tight, and there wasn’t enough height to properly clear the sights. Also, the trigger guard slot needed to be extended. The spare magazine holder, using only friction to hold the spare magazine, was very sufficient in terms of retention, though may later be assisted with the use of a simple Velcro strap attached to the sides. These changes were instituted in the second iteration, hereto referred to as V2 or Version 2.

3) Version 2 and Iterations

  

V2 concept.

In V2, an optional barrel extension with an adapter was added. This would allow the user to “swap out” different muzzle devices, such as extensions, tracer units, and a standard muzzle. These would be retained using two M3 screws installed into either side of the receiver. An additional point of retention to the airsoft pistol was also added – two screwholes on either side of the rear receiver group that mesh with the HiCapa’s grip screws. Here, we can also see the institution of a “traditional” gutter sight, and the re-design of the stock. The new stock design reflects more elements from the Sig MCX pistol-caliber carbine, while still remaining quite simple. This may be re-designed before the final version for increased aesthetic appeal and reduction in printing materials/time. Additionally, at the concerns of a friend for stock mount durability, the stock retaining brackets were extended.

At this point, I began playing with a rear grip extension, functioning to create a “thumbhole”- like component to the build.

Not a fan…

While the design did look a little tighter with the stock collapsed, it fell apart with the stock extended. Of course, more detail could’ve gone into the grip extension area, but I didn’t really like how the look was changing, so I decided to move forward without it.

While designing this kit, I’ve been in nearly-constant contact with a person who’s very strongly considering buying a copy of the kit once it’s finished and proven. I made some visualizations for him so he could see how it would look in his desired configuration and coloring.

He mentioned at this point that it was also very reminiscent of the Killzone Spec Ops SMG.

However, at this point, I wasn’t fully satisfied with two elements of the build: First, I wanted an under-barrel accessory slot; Second, the charging handle design seemed fairly weak.

V2 charging handle, V2 muzzle adapter, and V2/V3 receiver joining pins/brackets. Printed in Hatchbox White PETG at 0.1mm resolution.

Due to the nature of the one-piece forward receiver design, the charging handle would have to be printed in two or more parts. I had figured at the time that printing the charging handle in bilateral sections would result in the strongest product, but this was changed for the final version. Unfortunately, this join ended up being very weak, which caused the join on the female side to crack both times I printed it. While this would still work with chemical bonding (we will talk about the trials and tribulations of chemically bonding PETG later), and the crossbar itself should be quite strong when bonded, I decided that a better solution would be needed. However, due to the sheer strength of PETG in this application, I’m not worried at all about those thin “guide rail” tabs toward the rear of the charging handle. I can bend those tabs completely 90 degrees and then reset them to the proper position with no visible damage to the material. While this would work-harden the material and eventually break it over repeated abuse, it was an inspirational moment of the build process – that, and a confidence builder in the accuracy of my printer at this stage. PETG is printing at a much higher quality than even PLA on my printer.

Video demonstrating the flexibility and strength of the material used for these parts.

With this in consideration, I moved forward to address the problems.

3) Version 3 to Present

With the lessons learned from V2, I moved forward with design elements. I also added a new feature: a GoPro “fork” joint that can be replaced with a block of material if the mount is not being used.

Feature Creep at its finest.

It is retained with an M4 screw.

I also extended the forward receiver to accommodate an under-the-barrel picatinny rail mount:

Version 3 concept - finalization stage.

Due to the size of the rear receiver and grip extension sections, they needed to be bilaterally separated and have joins designed. A solid mounting method to join the rear and forward receivers also needed to be designed. To avoid the printing size restrictions, I separated the rear receiver and grip extension into two pieces each, and simply cut “puzzle piece” joins for each assembly. These would later be chemically bonded and sanded flush. To mount the rear and forward receivers, a series of 16 holes with matching “barrels” were cut through the rear and forward receivers, which would, again, be chemically bonded and sanded flush.

In addition to this, I needed to address the charging handle concerns. I approached this in two ways.

First, I extended the contact plate of the charging handle by several times. This gave it a more substantial amount of applied force, as well as more stability. Additionally, the charging handle existing in this configuration would reduce and side-to-side “wobbly” of the charging handle during pushback.

Second, I added a spring guide and “guide” section to the forward receiver and charging handle respectively. This would further control the charging handle while also allowing space for a spring to sit between the face of the HiCapa slide and the charging handle. This results in a fully non-reciprocating charging handle.

With these two fixes in place, I separated the charging handle assembly into four pieces – the charging handles on either side (with an inlay cut for mounting); a central “sled” (which was able to slide between the charging handle slots for installation); and the “loop” as its own piece (again, with slots for mounting). The intention is that the “sled” will be installed with one side of the charging handle already chemically bonded to it, then the second charging handle would be chemically bonded while the assembly is in the forward receiver. Finally, the “loop” would be chemically bonded to the bottom of the sled, resulting in one large piece of PETG for the charging handle. This resulted in a much more robust charging handle with less side-to-side play – a much-needed and highly-concerning aspect of the build.

Part 2 coming soon. All parts for the build are currently in the process of printing.

-Craig