Basic shield charms being able to stand up to modern guns just as easily as older style guns is more difficult to believe than, I don't know, any Transfiguration spell ever?
Well, okay, you're the guy writing the story. But canon basically disagrees with you.
Hard. Th energy behind the bullet doesn't matter to HP 'It's basically conceptual based' magic, what matters is that the shield spell line basically laugh at physical attacks and only start having trouble when its magical attacks.
The joke was about your spelling of wasting.
As for the Shield Spell
The
Shield Charm[3] (
Protego)
[1] was a
charm that protected the caster with an invisible shield that reflected spells and blocked physical entities.
[3] There were also multiple variations of the Shield Charm.
The Shield Charm's origins were unknown, but was in use by the
15th century.
[8]
The most famous use of the charm took place in
1484, in the town of
Poppleton, where the tyrannical
Earl of Paunchley held a jousting match. He insisted that all the nobles for miles around must come to cheer him on at the tournament and "
no excuse would be acceptable" for missing the event. But young
Edmund Gaddlegate fell out of a tree that day and broke his leg, and his mother put him to bed. The Earl wouldn't hear of anyone staying home, so he dragged Edmund and his mother to the castle, and said the mother had to watch while her child was tied to a horse and forced to compete in the jousting.
[8]
The other knights left the tournament "in disgust" at the idea of jousting with a small injured boy, so the Earl vowed that he himself would attack Edmund. But a witch in the crowd named
Hannah Cockleford had other ideas.
[8] Fed up with the evil Earl, she cast an extremely strong Shield Charm between Edmund and the Earl, who was "
squashed flat in his armour" when the Shield Charm knocked him backwards and his horse landed on him. The Earl of Paunchley did not die, but when he came-to he was under the impression that he was a
donkey named Hairy Cyril.
[8]
Now, Protego is basically the second biggest brother (Protego Maxima) of the general shield line, so you could argue lesser shield spells would have issues, but nothing we've ever seen from canon suggests firearms are an actual serious threat to a well trained adult wizard for anything but the wizard's own incompetence. And Protego according to the movies and games can be cast in several different ways, such as 'Shield', 'Wall in front of me', and 'Bubble shield'. The Shield format seems to be the beginner form, but the wall is also canon because Hermione casts it like that in the tent during Ron and Harry's argument in the 7th book. The only version of it we don't see in the books is the Bubble Shied format.
Other varieties of the Shield Charm included as followed:
