If you want additional accuracy without affecting FPS, your choice of rubber and nub combination is the thing to look at. If you think about how the system works, ordinary nubs are plain cylinders of varying stiffness which press the rubber down through the barrel window. So when the BB get's pushed past it by the compressed air, the top of the sphere is squished against the rubber. But the BB is not an exact tight fit in the barrel...
Decent BB's are manufactured to size 5.95 +/- 0.01mm & 6.01mm is a very tight TBB = approx 0.025mm either side as a best case. Not much, eh? Well if you imagine that slightly fat BB leaving the muzzle and moving 0.025mm to the left in, say, half its diameter 2.98mm, then @50m (50,000mm / 2.98mm = 16778.52348993289) that best case (x 0.025mm) becomes 419.463mm, ie compared to aiming at the centre of a standard man sized target 600mm wide, a miss.
It is much more complex, because rather than the maximum physical distance which a BB can move off-target within the barrel, how the BB spins when it is on its trajectory will be much more important to where it finally ends up. But consider the BB squashing past the rubber... what is to stop it being pushed completely to one side or the other? In fact, due to all the pressure involved, if there is any space the BB can move into, why wouldn't it? And it does. So now consider that the rubber is squidgy, so when the BB passes under it off centre, more of the rubber squidges down around the side opposite to the direction of movement off centre, which makes the hop effect off centre.
There are two methods to combat this which both work in the same way but, as far as I am aware, are mutually exclusive: H-nubs and split bump rubbers.
H-nubs (also called X spacers, X-nubs and fishbone spacers) work with standard hop rubbers / split bump rubbers need to be pushed down by ordinary nubs. The principle is that by controlling how the rubber squidges, it forces the BB into the centre of the barrel and the hop effect will be applied evenly.
H-nub /
gucci H-nub /
PDI W-hold rubber /
Falcon Dual Point rubber
You can't buy CYMA hop rubbers on their own, afaik, but many ppl look down their noses at such cheap stock parts - if you meet anyone who wants to upgrade their new CYMA gun with a new rubber, buy the stock one if you can. They have a simple split bump which opens wider the more it is squashed; it works very well.