Emiya Shirou was a pretty terrible magician. It all came down to his affinity being "Swords" rather than something normal like "Fire" or "Water". So most if not all spells failed and he was restricted to working with weapons. He was resigned to it. He would spend the rest of his life analyzing and reproducing weapons.

"Its too bad I don't have different Magic Circuits." he thought. With different Circuits Shirou would have more prana and access to a lot more spells.

"Man, Magic Circuits are like the ultimate, most powerful and versatile weap... on." Shirou froze. And slowly turned to look at Rin.

"Shirou, are you okay? You look a bit... wait! What are you- Kyah! Pervert!"

Shirou Scanned, Traced and Reinforced two hundred and thirty new Magic Circuits based on Rin's. The result was a massive increase in his prana and better yet the instinctive use of those Circuits as Rin would. He didn't know the theory but he could sure do the spells. And since he had almost twice as much prana as Rin he could afford to make mistakes.

Even Saber-chan commented on his new found power!

Saber-chan, who was actually Arturia. Who came from an earlier Age. Who didn't have a normal Magic Circuit but a Magic Core. Hmmm...

"Saber-chan, stay still will you? I want to try something."

"So you're me from the future?"

Archer opened his eyes to stare at his younger self in disbelief. How did...

"That's an awesome Reality Marble you have there. Mind if I copy it?"

"But... You... How...?"

"Although, doesn't that mean you already know how this thing goes? I mean, can't you use your knowledge to manipulate events? And then there's how you already know us and stuff... Did you happen to memorize any lottery numbers before coming back?"

Archer gaped at his younger self before noticing Tohsaka Rin in the background. Fuming. Rin never did take kindly to being played. Maybe he should put his Independent Action into use?

"Get back here traitor!"

"So that's how you do fast incantations. Awesome!"

Caster did not jump into the air from the shock but it was a close thing.

"What the... How did you get in here?!"

"Fun fact about layering fifty hiding spells: they count as Presence Concealment. By the way, that's a nice dagger. Mind if I have a look?"

"I am Gilgamesh, the King of Heroes!"

"And you don't have anything good on you. Let's go Saber-chan."

"How dare you insult me? Let me demonstrate the folly of your thoughts with me Gates of Babylon!"

Gilgamesh casually gestured as countless weapons left the Gates behind him and launched themselves at... at...

Gilgamesh turned to look at his Gates spewing forth weapons that disintegrated the moment they left.

"What the?"

"Now that's what I call a collection. Got anything stronger?"

"Normally I would reserve this for my Queen but your insolence cannot go unpunished. Enkidu!"

Chains materialized from thin air and bound Shirou securely. He struggled, he dematerialized them, he tried to cut them but he could not.

"Very nice trick Golgy."

"What. Did. You. Just. Call. Me?"

"Hey, Saber-chan, some help here?"

"Excalibur!"

"You!" the old man growled in a menacing voice.

"Who me?"

"There can only be one punishment for messing the timeline as thoroughly as you did boy! And that is..."

"BANISHMENT!" the second Zelretch intoned as he completed the spell.

"Man, am I glad that boy is not my problem anymore." the Dead Apostle Ancestor said while shaking his head.

A/N: The idea here is that Shirou figures out his casting problems early in the War and the proceeds to munchkin them. Zelretch banishing him is just to get him to whichever other setting you want him really.