To achieve a hardness of 260-270 BHN in aluminum bronze, the chemical composition should be as follows:

- Aluminum: 8.0 - 9.5%

- Copper: Balance

- Iron: 1.5 - 4.5%

- Nickel: 3.5 - 5%

- Manganese: 0.5 - 1.0%

This composition provides a good balance of strength, hardness, and toughness. The aluminum and iron content contribute to the hardness, while the copper and nickel provide strength and toughness. Manganese is added to improve the alloy's resistance to corrosion and wear.

It's important to note that the exact composition may vary slightly depending on the desired properties and the specific manufacturing process.