Males of average height have about 4 grams of iron in their body, females about 3.5 grams; children will usually have 3 grams or less

300 * 3g = 900g.

300 * 4,5g = 1350g

https://en.m.wikipedia.org/wiki/Longsword

and weighing approximately 1 to 1.5 kg

It’s close enough, I’d say. OP’s maths that is.