Deathwings said:
Okay, specifically, all you need to Project is a perfectly accurate mental Blueprint. So, theoretically speaking, Projection can be used on ANYTHING. It's just that you need a PERFECT Blueprint, so good luck memorizing one to such a degree without something to cheat, like UBW.
Perfectly accurate blue-prints could theoretically come from Structural Grasp, which is a spell Shirou has for understanding anything that isn't a weapon. It's how he analyzes the weaknesses and malfunctions of the machines he repairs.
But UBW-projection is an entirely different beast than Gradation Air. They are only superficially similar. It's like comparing an electric-rail bullet train to a steam-engine. Completely different innards and outputs.
Shirou's projected kettles were completely different from anything Rin could make, being more perfect and complete and thus able to continue existing on their own... at least for a while. The difference was not Structural Grasp, but UBW-projection.
That's also why Shirou progressively lose the ability to Project Avalon : he started with an utterly perfect Blueprint that he progressively forgot because UBW couldn't stock it since it's not an armament.
Tell that to warriors who fight with their sheaths. Given that it is a defensive armament and a super-hard sheath, Avalon should qualify as much as Rho Aias.
However, I'm willing to accept the idea that Avalon does begin to fade from UBW based on the fact that it is Rank EX, which is something normally beyond Shirou's ability to concieve or reproduce. Or, if Avalon continues to remain in UBW, Shirou cannot project a perfect copy of it, and only an imperfect copy appears when he expands his reality marble.
(Incidentally, I favor the idea that expanding UBW upon the world automatically creates one copy of every weapon recorded in Shirou's soul. For free. Any additional copies cost the same as normal.)