Titanium alloyed with Vanadium and Aluminium is actually used to build stuff. They are used in aircraft, high zoot bicycle frames and of course, guns. Ti is about 1/3 the density of steel but just as strong. It is also expensive.
Scandium is a very rare, very expensive metal that is added to aluminium (only in very small quantities) to make a very strong alloy. So when you buy a Scandium gun , you're actually getting an aluminium alloy gun. Since Aluminium is only abt 1/2 the density of steel, an all Al gun would be very light indeed.
I learned all this when I was shopping for a new bicycle last year.