It's not a matter of the Bible being an instruction manual; it's a matter of it being (or it being made out to be) a source of guidance for humanity. It doesn't have to give detailed instructions on how to build a car to discuss the moral and ethical implications of auto use, for instance.
And there's other strangeness... for instance, why is it that Revelation - a book made out to be a collection of prophecy that describes plenty of things beyond the ken of someone from the first century - mentions things like swords and trumpets but no objects that have been invented since then?
The Bible is a source of guidance for humanity - that humanity has a very serious problem with God and people will either perish eternally for that or they have their sins paid in full by Christ. And, through the Bible, people come to know their Creator, why they were made, where they are going after death, why the world is a mess and so on. Yes, it is a guidance, but it has to do more with things (those of God) which transcend objects that rust.
As 4consideration accurately pointed out, it would be unwise to use terminology which the people at your time don't comprehend. Nonetheless, I was referring to the terminology being incomprehensible, not the concepts themselves. How in the world can our limited, finite minds understand God fully? Do we really comprehend how God is three Persons in One: the Trinity? No... but that's the point. Since God is superior to us, then
He's the One Who comprehends us fully.
Let me illustrate how ridiculous your assertion sounds.
"From His mouth comes a sharp
sword, so that with it He may strike down the nations, and He will rule them with a rod of iron; and He treads the wine press of the fierce wrath of God, the Almighty." Rev 19:15
Does it sound better like this: "From His mouth comes a huge
nuclear bomb, so that with it He may strike down..." ?
The book of Revelation is full of metaphors and it tries to explain something that has never happened before, in words that were comprehensible. The sword, for instance, is a symbol of strength, justice, even aggression. But the ultimate reason for the book is linked to God's hand is the unfolding of history, so the emphasis is not on words like "sword" or other human inventions. In addition to that, it would have been improper for John to write in an obvious manner, which even the Romans (who were not accustomed to Hebrew theology and terminology) could understand. He had to write in a coded fashion, as arthra mentioned. I highly suggest
BibleOrTradition's videos on the Book of Revelation. He also commented that it would have been unwise for John to write about the downfall of the Roman Empire in a way that even the Romans would have understood.