Well, actually, FMJ does increase damage, however very slightly. Look at the damage bar when you add FMJ and you will see the bar with a little green at the end showing a bit was added. Silencers don't really reduce damage, they reduce range. Now because the bullet is traveling slower and can't get as far, that is why it doesn't do as much damage, so it does decrease damage but only long range. True with that last part and true that adding FMJ does not make it instant kill.