how can i get the camera projection matrix out of calibrateCamera() return values

calibrateCamera() returns you
a 3x3 matrix as cameraMatrix,
a 4x1 matrix as distCoeffs, and rvecs and tvecs that are vectors of 3x1 rotation(R) and 3x1 transformation(t) matrices.

What you want is ProjectionMatrix, which is multiply [cameraMatrix] by [R|t].

Therefore, it returs you a 3x4 ProjectionMatrix.
You can read OpenCV documentation for more info.