Yet another help request

[cur]

Hello guys. I will try to keep it short. I am a total beginner who would like to learn to build a pneumatic semi-automatic BB gun as a self-defense weapon, purely for educational purposes. And this is not a sarcasm, as I am an engineer who loves to dig in ideas just for fun. I came across several videos where people built air rifles that are formidable, and I was wondering how one builds one, which ended me up here. As many have suggested, I have browsed the forum for help and to find what I am looking for, but most of the links are not working anymore, and the ones that are working are either too complicated for me or some are too little documented. After 6-8 hours of reading here, I decided to start my own topic, and I can compile the things that are going to be written here, for future reference. So here I go:

I would like to learn to build a semi-automatic BB gun that operates with enough power to damage a "realistic" target. The first questions:

- How do I 'start' designing these things? Is there a drawing software? I have seen couple of options, such as Google Sketchup and ... Gun Design Tool (I forgot the whole name unfortunately).

- Is there a resource to learn the designing part?

Next, for pneumatic cannons, Spudfiles Wiki has the following:

• Compressed gas storage reservoir (chamber for short)
  • How can I determine the pressure capability of a chamber? Lets say I have determined the capability of a chamber. Then, how can I verify that the entire system will tolerate the pressure range of the chamber?

• Filling valve (bicycle Schrader valves, quick disconnects, etc.)
  • This part looks the easiest as the idea is straightforward.

• Firing valve (ball valves, sprinkler valves, piston valves, bolt-style valves(see piston valves) quick-exhaust valves, and burst disks are the most common)
  • Now, my question is here. After hours of reading, I understood how QEV valves work. The problem is that I never figure out how these QEV rifles are semi-automatic. In other words, how the gun reloads itself automatically? There is a gif associated with 'Caselman' type guns which seemed very nice, but I cant wrap my head around it. It looks extremely delicate and complex.

• Barrel
  • I would like to learn building something small, like .177 perhaps. The key is for me, again, I would like to learn building a semi-auto, and from what I have read, the barrel is also important when the design is semi-auto, and barrel must have according modifications to receive pellets.

• Pressure gauge (for safety & repeatable launches)
  • No need required on this part

I am sorry if this became bulky and complex. In essence, I would like to learn to build a QEV Semi-auto .177 BB gun. However, the topic seemed so vast, that although I kept reading for a good amount of time, I couldn't figure out how/where to start and some of the things went above my head.

Any help is much appreciated!

[jackssmirkingrevenge]

Welcome to the forum! That's a lot of questions and indeed if you're completely new to the subject the amount of information here can be overwhelming.

a pneumatic semi-automatic BB gun as a self-defense weapon, purely for educational purposes

It's worth noting that this forum frowns upon any discussion of shooting animate targets, and rightly so, however "enough power to damage a "realistic" target" is a good enough design criterion.

A good question to start with is what is your budget and what tools and materials do you have access to?

A QEV semi-auto can be made with mostly off-the-shelf parts requiring more assembly than fabrication, but as it essential dumps the firing chamber for every shot, it is a very inefficient design. This means that in order to get decent power of of it, then it will be big and bulky. A hammer valve that only dispenses a "sip" of air from the primary chamber is a better idea, I've made that case here, but it is more difficult to build especially if all you have in your workshop is a vice and power drill. Let us know a bit more about your situation and the advice you receive will be more pertinent.