Hello all,
I've been working on a custom pump action shotgun prop to be used as a film prop. Designed in CAD and 3D printed, it's based off the Ithaca 37 pump action shotgun but built from scratch as a model gun. It has the usual model gun safety features such as plastic construction and a metal chamber insert to prevent a live round from being chambered. Barrel is also permanently fixed to the receiver. The bolt does not lock and requires the user to keep the fore-end/pump held forward. The bolt also has a 'firing pin' that is soft and not pointed but flat at the tip, so just like in model guns, its purpose is to push against the 'primer' insert in the base of the cartridge.
As part of the process, I had to design my own cartridge from scratch. I say design, but I based it off the C-Tec replacement cartridge that is available in Japan for the MGC M31 shotgun replica. 3D printed prototypes shattered after a couple of shots so I instead turned to the lathe to make the components. This is the design. I didn't like the original MGC M31 design that is one solid piece and has the caps inserted at the tip of the cartridge, even though they're much simpler. It just wouldn't work for this particular shotgun replica design.
Grey parts are aluminium including the base but I did make the primer out of steel because for some reason aluminium may have been too soft to get reliable detonation of the cap. Brass might work but I'm out off raw brass round bar. The base screws into a plastic hull that I turned on the lathe from HDPE stock. The front end is hollow to allow the cartridge to slip over the chamber insert. The insert is kind of like the detonator that is found on most model guns but it doesn't actually have anything to do with detonation in this design. It's purely an insert to serve as a permanent obstruction that prevents live cartridges from being fed. The insert also has a narrow channel drilled down its centre with some holes around the front part to allow smoke and sparks to exit.
I apologise for all the crud covering the components. I've been doing a lot of testing with caps and I didn't have the patience to clean between tests. No blowback needed for the operation anyway so friction is not a concern. However, I did add an o-ring to the primer for a better seal so some grease on this helps to free up movement and increase the likelihood of the cap going off. I started by testing this cartridge out with the cheap plastic toy ring caps and more recently, thanks to Claymore from this forum, I have been able to procure some 7mm Marushin Caps. If you look at my rough technical drawing, you can see the red bit is the cap but there is a cavity at the front that can accept a second 7mm cap, in a reversed position. The video clip below shows how the cartridge performs with 2 caps loaded.
Cap detonation is sporadic so not 100% reliable at this stage. The mass of the hammer, which is 3D printed, along with its spring is probably to blame for this. I did increase the mass of the plastic hammer by drilling into it and adding some metal inserts secured with epoxy and this helps.
Video link, there is sound too.
https://imgur.com/HNSWUDyI apologise for the poor video quality. I filmed on my phone vertically and had to crop it so the resolution took a hit. I did jump a bit because I edited out the first attempt where it didn't fire so I wasn't really expecting it to go off this time. Ignore the yellow bits, I ran out of black filament for my 3D printer. Eventually, I am going to sand everything down and smooth out the surface for painting anyway. I will show off more of the shotgun when it's finished. It works though, with feeding, firing and ejection, The white stuff on the barrel is where I have had to repair it over the last few weeks because I was experimenting with flash cotton and used too much which...You can guess what happened.
3D printed parts are inherently weak due to layer lines and the PLA plastic. They're suitable for replicas including model guns/cap guns but there is literally no chance that this could withstand high pressures, for the record. I run a film company and I'm trying to build suitable props for production and a pump action shotgun was something that I needed.
Wed Sep 25, 2024 10:24 pm
