Why should it be illegal to build a weapon? Slingshots can be quite deadly, you know. I'd draw the line at weapons that can be used to kill or hurt large amounts of people... those, you should need a special permit for, up to a certain point... and maybe keep the weapons at a secure location (I'm thinking M-60s here). OFC, nukes are out of the question. If you want to build those, go work for the government.
Exactly where would you like the weapons ban line to be drawn? Slingshots are OK? Crossbows & throwing knives? CO2 pellet pistols? Air Rifles? .22s? What? Because you know they (criminals) will just take it to the next level, until you make it not worth their while to build weapons, by enforcing stern punishments for crimes.