Gun metal contains

Gun metal, also known as red brass, is a type of bronze that is commonly used in casting and for making items like valves, gears, and bearings due to its resistance to corrosion and high strength.

It typically contains 85% to 92% copper, and the remainder is made up of tin, with small amounts of lead and sometimes nickel to improve its machinability and corrosion resistance.

Now let's look at why the other options are incorrect:

Option A: 70% copper and 30% zinc — This composition is closer to brass, not gun metal.

Option B: 90% copper and 10% tin — This is a typical composition of standard bronze, not gun metal specifically.

Option D: 70 - 78% copper and rest tin — This composition does not accurately reflect gun metal and lacks the small additions of lead and nickel that characterize gun metal.

Conclusion: The correct composition of gun metal is 85 - 92% copper, with the remainder being tin and small amounts of lead and nickel.

Hence, the correct answer is Option C.