Being able to hand-draw your weapons sounds like an awesome idea! It would be difficult though.
A simple method that comes to my mind is to use a variable such as a Vector2 List, and record where your mouse was drawing. So for each line you would store two Vector2 positions in order to keep track where the line starts and ends. Then, during the game, you would "redraw" the weapon and offset it depending on the player location.
A second method is to use image files that you put into the game. The person playing can draw a weapon, whether that be in an in-game editor or on their own drawing app. Then the drawing would go into the game files so that the game can find it and use it.
Third method is not quite drawing your weapons, but a bit similar. You just pre-draw several different gun parts, then allow the player to switch between them freely. With enough parts, the player can still design whatever they want, and it would make stats really easy to make. I've actually made a small game that uses this method, so I can confirm it's easier than making a drawing editor.
It all depends on what exactly you have planned. The first method is easy for the player but might be hard to make. The second method gives a ton of artistic freedom, but can be confusing for the player to use. The third method is really easy for the player, but takes away the freedom to draw.