The other huge issue you have is that this is an extended test. the rules on page 48 give you the guidelines. But, they are only guidelines.
Here, you have to find out what exactly the player wants.....
If he wants something like: "I just wanna design and build my own gun, you know, like the Predator 4, but not.." would put it in the "average" difficulty (he could use the Predator 4 as a baseline template, then make his changes)
If he wants something like: "Humm, Maybe a light pistol sized heavy pistol that fires full auto, and has a 30 round clip" would probably put it in the "extreme" category.
If he wants something like: "I wanna design a pistol, that fires AC rounds, in full auto with 30 round clip, and can shoot fireballz!!!" you slap him.
Then you have to decide on the interval. Well, you realistic choices a start at an hour and go to a month. An hour is not really realistic at all, so the other options are more likely. A month is probably the best interval if he has to do everything from scratch. (design the schematic, retool the equipment to the specs, fab the parts, assemble the parts, test).
so you are looking at a threshold of 12 to 30+ with an interval of 1 hour/day/week/month VS Logic + Gunsmithing (or whatever skill you feel allows him to design and build his own gun).
Keep in mind that during that interval, they can do nothing else of significance. (so no runs)
Also keep in mind that places like Ares have entire labs filled with engineers who make teamwork tests to do exactly this thing and they still fail (*cough* Excaliber *cough* ).