Slingshot has the fastest projectile, right? Just give the Long-range class (SN!p3r) a specific damage boost and use some devices to set ammo before reloading to 1. There are guides on custom ammo and reload speed. I’ve seen them.
PS: I don’t really know much about TF2.
2nd PS: I’m feeling a bit better compared to this morning. Today is the 2nd day of end-of-Term testing, so I have had a lot of time to rest and tomorrow is the first day of Winter Break! 
And I spent so long typing this the headache pain stopped until I zoned back in XD.