Simplify the following text for a Grade 6 native English-speaking person.
Original: The safari heyday of the early 20th century ` Nitro era ' records much literature on such calibres as the .577 Nitro Express , .375 H&H Magnum , .416 Rigby , .404 Jeffery , .505 Gibbs , .450 Nitro Express , .470 Nitro Express and many others .
Simplified: The safari heyday of the early 20th century ` Nitro era ' records much writing on such calibres as the .577 Nitro Express , .375 H&H Magnum , .416 Rigby , .404 Jeffery , 505 Gibbs , .450 Nitro Express , .470 Nitro Express and many others .