Have a gun which works on a small quantity of high pressure air rather than a larger quantity of lower pressure. That means that when it expands and exits the barrel it will be at a lower pressure.

Use heavy pellets so they take more energy out of the air.

Use a .25. The air will be at a lower pressure when it exits as it has filled the larger barrel. .25s rock.

All this assumes 12ftlb energy output.

Or just detune to 6ftlb- so you need less air in the first place.