Aliens M41A Pulse Rifle - The Stock

The stock of the Pulse Rifle is, at its core, a piece of metal u-channel with a slot pattern cut into it. The stock extends forward under the rifle's body work and it's mounted to the top of the Thompson's upper receiver.

It took me a little while to realize the slots in the stock are not purely ornamental. The slots on the top are close to lining up with the holes where the Thompsons rear sight use to be attached. Our Stock will be designed to be bolted to the Thompson through these holes.

 Reference model on the left. Measured model of the back of the Thompson on the right.

Reference model on the left. Measured model of the back of the Thompson on the right.

To verify my measurements of the Thompson, I printed the test objects I created in the last step and fitted them onto the gun's body. I had to modify the 3d model a bit and reprint a few times to get the fit I wanted. It was time well spent, now I know my 3d model is accurate and how much allowance I need to incorporate between the Thompson and the final printed parts.

I doubled the thickness of the stock's walls in comparison to what's in the reference model. The extra thickness will add strength to the stock and make it less brittle, a critical improvement for a part 3d printed out of PLA. The cutout in the right front of the stock lets the Thompson's charging lever move freely. The inside of the stock, the part that touches the Thompson, is shaped to match the geometry of the test piece I printed earlier, so I know it will fit in place properly.

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The final part is a 278mm long and will barely fit onto the 285mm x 153mm build area of my MakerBot Replicator 2. To add strength to the finished part I set it to print with 4 outer shells and 50% infill.  I also turned the stock to print at a slight diagonal, so that the layer lines on the flat surfaces wouldn't be running parallel to the direction of force.

The final printed stock fits marvelously. Its not so tight that its deforming the plastic of the Thompson, but there is good surface contact on all sides. Ther charging lever can move freely into the slot without any problems and the higher print density made the part really strong.

For now, i just used two bolts to hold the stock in place. They pass through the slots, through the sight mounting holes, and into the upper receiver where nuts hold them in place. There are many advantages to this over just glueing the stock on.  I can adjust its location, remove it for sanding and painting, and easily replace the stock if it gets broken.

Next up, The hard part, building the primary body components!