From a mechanical standpoint, your best bet is to start with a known-working model, then design parts to mimic those functions. Frequently, the only reason for things to be designed as they are is because we didn't have 3d printing until recently, and subtractive or molded/cast manufacturing can't produce things with the same detail. Start from a working model, redesign.

And to those implying that it's a matter of "wanting to kill people", allow some clarification: Most of us don't *want* to kill people. But when you live in a dangerous area or you have valuables, you need to be able to protect yourself. And when your only options are a half-brick in a sock or an extremely expensive gun, the problem comes down to money. Some of us are just broke. I'm fine with a .22. As a smaller person, a .38, a .357 mag, etc are just too unwieldy for me. I LIKE smaller weapons. I'd rather stick to my blades, personally, but if someone attacks me with a gun, even Ghandi agrees that it is right and just that I defend myself with my own gun.