Collapsing Stock For a Custom Crosman 1322 – Part 3

In my first and second posts on the subject, I only really discussed the metal parts of the collapsing stock for my custom Crosman 1322. Those are important, of course, but we’ve left out the part that actually touches the shooter – the shoulder pad. I chose to 3D print it, so I would have a lot of design flexibility. That worked, but I made some mistakes along the way. I learned a lot, and maybe you can learn something too!

What’s the goal?

I want something that is lightweight and weatherproof. It has to be strong enough to survive the rigors of travel, but since it’s on an airgun it doesn’t need to stand up to recoil.

Why plastic, and why 3D print?

A comfortable rifle stock incorporates some curves. My lathe is great at making circles, and my milling machine makes straight lines really well. But compound curves are another matter. In theory I could hand-shape something in metal or wood, but that didn’t feel like the project I wanted.

Plastic, on the other hand, is weatherproof, and fairly light. A 3D printer can make any sort of complicated compound curves with no more effort on my part than if the lines were straight.

But mostly I wanted to use my 3D printer in this project. As much because I could, as for rational reasons.

3D Printing Nerdy Details

Just kidding. If you want to know that sort of stuff, just ask. But I’ll spare the uninitiated from most of the jargon here.

I will mention that I printed all versions of the shoulder pad in a material called PETG. It’s based on the plastic that soda bottles are made from, so it’s light, tough, and very slightly flexible. It also will stand up to temperatures near that of boiling water. So if you leave it in a vehicle in the summertime it won’t transform into the Salvador Dali version of itself.

The Mark I Shoulder Pad

I kept my first attempt pretty simple. I’ve never had any formal training at CAD modeling, so I’ve been trying to learn as I go. I knew the general shape I wanted, and I knew I wanted reinforcing ribs from the sockets for the rods down to the bottom of the pad. The initial model took a lot of time to make, but I learned a lot. Mostly I wanted have some sort of a design, and build it, so I would have better ideas for the next version. I fully expected it to fail, but I had to start somewhere. So I set to work in Onshape.

In a sense it worked. Mark I turned out to be far too flimsy. The sides cracked during handling. It also turned out to be a little short. Back to the drawing board!

The Mark II

For the Mark II design I set out to fix the problems I knew about. I beefed up the walls, and made the pad longer from top to bottom. Then I added a bunch of reinforcing ribs between the outer walls and the reinforcing ribs.

I liked the result better, but I still saw problems. The rods were just a slip fit into the holes in the pad, so getting them aligned with one another was difficult, and keeping them aligned was impossible. Also, I could see that all those little reinforcing ribs were going to catch a lot of dirt and moisture in the field. Finally, it was awkward to reach the pistol grip when the stock was fully collapsed. It was possible, but it would seriously interfere with good shooting. That wasn’t a huge deal, but if I had to redesign anyway, I would try to address that too.

Mark III, Designed but not printed

For the Mark III design I wanted a solid shape, and a positive attachment between the rods and the shoulder pad. And hopefully improve access to the pistol grip when the stock was collapsed.

For the attachment, I chose to drill and tap the ends of the rods. Then I could just put a corresponding hole in the shoulder pad.

I didn’t take pictures. It was a pretty quick job on the lathe.

Designing this version was a bit challenging. I vague idea of what I wanted, and I had to learn how to make the modeling software create the sort of shapes I wanted. This was the result.

I didn’t like the look of it – it stuck up too far above the rods. Something about it made me think of the battle droids from the later Star Wars movies. For various technical reasons I couldn’t move the holes without redrawing a whole bunch of other elements. So I didn’t even print this one. I just abandoned it and moved on.

The Mark IV

I took some lessons I learned from the Mark III and tried to apply them to the Mark IV. Mostly these were lessons about how to use Onshape, but I did want to further narrow the bottom of the shoulder pad to improve access to the pistol grip.

So far I like this one quite a bit.

I considered adding a compartment for an oil bottle, but better sense prevailed, and I declared that enough was enough. I had achieved all my design goals, and the result actually works pretty well. If I decide later I can’t live without the oil bottle I’ll deal with it then. For now, I’ll enjoy shooting the rifle.

Here’s a shot of all 3 generations of printed shoulder pads.

Thanks for following along on this slightly silly project!