From Exterminatus Manual
The Imperium equips it's marines with the best weapons technology from a million worlds according to the rules of the Codex Astartes laid down by the Primarch of the Ultramarines ten millennia ago. Only weaponry up the the standards of the codex are are marked "Chapter Approved", anything less is heresy.
Tyranids are grown together with their weapons, creatures symbolically linked to their flesh, so they think and fight as one. Equipped to kill from birth. Tyranid weapons grow their own ammunition, which means they are generally slower between shots, however it also means they never run out.
The weapons in EX are not 100% accurate, shots deviate somewhat from where your cross-hair is pointing. Some weapons are intrinsically more accurate then others as mentioned on their pages. The accuracy of the weapons is also effected by the behavior of the player. Jumping or sprinting for example will make your shots wildly inaccurate. Crouching down steadies your aim and improves your accuracy.