Hello, I have the answer, generally, in 20" barrel.
http://natoreloading.com/subsonic556/
About 6 grains of Blue Dot using 55 grain, or about 6.1 of AA#7. This is using 55 grain bullets. this will get you close if your using 20" barrel.
Don't forget to shred up some cotton and put it in to hold the powder at primer end.
P.S. the page is called subsonic, but since there was zero load data for those powders, it was all over the place. But enough was done to understand what load will get you close to subsonic, or 1500, or etc.