It depends on the kind of projectile. If it's a simple firearm, I wouldn't use a sphere, cause if you use the sphere with a realistic speed (eg. 300m/s for a 9mm bullet, or up to 800m/s for .308 rifle ammo), it will not be very useful for collision detection IMHO. In this case I'd use a linepick that "hits" the target immediately, which is much more realistic.
If it's a rocket or some other thrown object, you can use any mesh, and simply use the right collision mode, eg. sphere to poly. But as ross said, as soon as there are two moving things (bullet and target) and they move towards eachother, the collision will be missed unless you do an "updateworld 0" after every entities movement.
If it's a rocket or some other thrown object, you can use any mesh, and simply use the right collision mode, eg. sphere to poly. But as ross said, as soon as there are two moving things (bullet and target) and they move towards eachother, the collision will be missed unless you do an "updateworld 0" after every entities movement.