800X as Greg says will get the job done & may presently have the most low-pressure load data readily available.
I'm of the opinion that Long-Shot could also make for a good low pressure powder in some 12 ga. applications, but there is scarce data available for any low pressure loads using it. Almost all easily accessed load data is for high velocity loads w/attendant higher pressures. You can have loads tested for both pressure and velocity at reasonable cost these days, so it could be a good avenue to pursue.
