Don't. A 209 is 10x the power of a #11 and ignites the whole charge instantly, giving a huge pressure spike. M/L are designed around 8,000 ft lbs of pressure. Modern rifles, shotguns and in-lines are 35,000 to 50,000 ft. lbs. The dastardly 209 system on a m/l is intended to ignite a solid propellent pellet, not loose grains (especially honest blackpowder).
If you're getting bad ignition, swap for a new nipple, make sure the hammer is hitting it square, make sure the channel is clear and free of oil before the first load, switch brands of cap.