At the start of 2020, I build a quadrupedal mech armed with an airsoft guns. Because I was aiming for a compact design I opted to design my own airsoft gun. The design I used back then had the muzzle velocity too low and the accuracy too poor, so the project was shelved. I recently had the time to give it another go, and ended up with an even worse performing gun.
"That doesn't look like an airsoft gun" I hear you say. Well, it's the simplest prototype of the mechanics I could design. There's no trigger mechanism, no gearbox, just the bare minimum proof of concept. A typical airsoft gun uses a air chamber with a spring-driven piston that forces air through the chamber, so that's what I made.
The original design used a rubber band to accelerate the BB pretty much
directly. This means that the BB velocity is equal to the rubber band
velocity, and that any motion in the plunger that pushes the BB is
transferred into the BB. Here's a video of the original:
It achieved 25FPS - I wanted 100FPS, and it had an angular shot dispersion that meant you couldn't hit a 5cm grouping at 1m. Clearly insufficient for any actual use.
In contrast, the new mechanism uses the rubber bands to compress air, and the air accelerates the BB. Once again I went for a fully printed design, and drilled out the barrel and sanded the plunger smooth so that the fits were better. Initial tests were promising, with me being able to blow a BB out the barrel to a range of a couple meters, but after attaching the plunger and powering it with rubber bands, the muzzle velocity was far far to low. The shots barely made it more than 2m.
I think the seals are just too poor. The seal between the two halves leaks a bit of air (though you can blow the punger out), and the seal around the plunger also leaks a fair bit despite the vasiline I added. A lot of this is likely down to layers lines and the poor job of bridging my printer did, so it's possible a reprint would improve things, but the fact that it is far worse than the previous incarnation is a bit demotivating.
I have a couple ideas, ranging from improvements to the original design to a hexagonal air-chamber (3D printers can prints straight lines more accurately than circles). I may try a plain-old reprint at some point with different slicer settings, but I think that will be another day.