580.
Trying to figure out what cartridge you are describing. At 36 grains this would be a very light .223. At that weight I would anticipate that round moving at 3700 fps.
I took for my calculations, if I remember, a 65 grain bullet moving at 3200fps. Of course we are talking yesterday and I was indulging so I am no longer sure. My point was this. Folks want to limit the lethal force any of us can carry into public. Folks are stuck on the standard AR 15 platform, because, well, media indoctrination trumps knowledge. Not wanting to fight that, and going with the flow I made a presumption. The presumption was lets reduce the standard mags capacity from 30 to 20 rounds for that configuration. This gives us the progress many desire, less lethality readily available walking around in public. We then, quite artificially declare the amount of lethal energy in that weapon the maximum amount of lethal energy we will tolerate. We then extrapoloate that to other calibers to set magazine maximums for those weapons. We declare we will tolerate, as I said, about 40,000 joules of lethal energy or 36,000 foot pounds, nothing more. Then, because we still don't want spraying of bullets, and because the .22 is uniquely low in energy we also have as a maximum mag capacity, regardless of energy, 20 rounds.