This app was mentioned in 2 comments, with an average of 1.50 upvotes
If you want a simple method (for mobile phones and tablets only) then Simplector or the Pro version is probably worth your time. InkScape is a much better programme if you know how to handle it but is still lacking. Both of them produce slightly bloated SVG images, but the Simplector ones can more easily be hand optimised. A somewhat complicated image can be compressed heavily [as seen here](/r/CrossView/comments/3cf10g/z/csuvl2x).
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Remember that they often take longer to render than more conventional methods and take more to create. Support is varied and only guaranteed on (mainly) newer browsers. The uptakes are that they're infinitely scalable (never lose resolution/clarity) and tiny file sizes.
https://play.google.com/store/apps/details?id=plasma.vector.editor.app
svgベクター系のなんちゃってIllustratorとかなんちゃってInkscape的なのあってるかも銃の正確な形かくならドロー系よりもいいと思う。