How to get Angle of velocity to rotate a bullet?

I have this:

CGPoint vel = hudLayer.rightJoystick.velocity;

            CCBullet* sp = [CCBullet spriteWithFile:@"g开发者_JAVA技巧reen.png"];
            sp.position = player.position;
            [self addChild:sp z:-10];

            vel = ccpMult(ccpNormalize(vel), 300);
sp.rotation = //how to get the rotation out of the velocity?

Any help?

float angle = atan2f(vel.y, vel.x);