First of all, you need to check the Properties of the projectile under Geometry first to make sure its mass is accurate and the same as the one used in your hand calculations.

If your hand calculation assumed that the projectile is a rigid body, it is possible to find that the predicted velocity is lower than the hand calculations. In Explicit Dynamics system the default behavior of the projectile is flexible so it can deform and develop stress. Some of the external work done by Pressure or Force boundary condition can be taken by the internal energy (stress multiplid by strain) of the projectile.

To find the rigid body velocity of a projectile, users need to right click on Solution Information before running the model and then click on Insert -> Total Mass Averaged Velocity. See attached picture below. This should give you the average velocity of the body, rather than the velocity at any arbitrary node. This is the velocity that should be compared with hand calculations.