But silencer decreases penetration (So I've heard) and FMJ increases it, wouldn't they basically cancel each other out?
The silencer only shortens the range of damage for most guns, i.e. - the ACR with silencer will do 20 damage at a slightly closer range than without.
With silencer and fmj, you can shoot through a wall for quite a while, and the enemy won't know where the hell you're shooting from, since you don't show up on the radar, and your bullets just make the weird bullet penetration noise. basically, the enemy just scrambles around trying not to get shot xD