QUOTE (Yerameyahu @ Sep 26 2010, 09:38 PM)

It's not 'just an extra test', though. It's months of work, especially for someone working alone; then, having parts custom-made (even in the nano era) isn't insignificant, and then you can finally put it together.
Why bother? Modify existing guns, or build existing (and somehow non-proprietary) designs. Be sure to destroy every last nano-tag in the materials you used, by the way, so you're not tracked down.
He asked about Shadowrun, and that's what fits into the setting and the rules. Do whatever makes it fun for you, but be aware of the whole situation.

Not quite True there, Yerameyahu... It does not take MONTHS of work to design something new, even working alone...
When I was younger (in 1982), I visited my Uncle, who is a Machinest, and who did custom work for Colt Firearms in his small Workshop/Facility (He was self-employed)... he had, among other things, a Programmable Industrial Milling Machine, Programmable Drill Presses, lathes, etc... as well as significant woodworking tools (Probably close to 1/2 Million USD in Equipment). One morning, because we were bored, we decided to
make some custom firearms, completely from scratch... they were to be Black Powder weapons, so not all that complex to design... took us a couple hours to design the weapons, and program the machines to cut the required metal pieces...while they did that, we fashioned the wood pieces by hand, and then spent the rest of the weekend fitting the pieces, finishing the weapons and testing their functionality. They were functional firearms, and a lot of fun to build. It took all of 2 days, including desing, programming, implementation, and fitting/testing...
It is not all that hard to do the same thing in Shadowrun... Some design skills, metallurgy, and Armorer and you should be just fine... I would enforce the minimal requirement of a Facility (By Shadowrun Rules you need access to a facility to design/test) to proceed, but it should be possible to do so without too much difficulty...
And Yes... If I were to do this, I understand that it would be a House Rule... but, it is not all that difficult to do, if you have the know-how and the equipment to do so...