Using a dummy projectile like that is probably more trouble than it's worth. It'd work better to either lower the spawnheight in A_FireCustomMissile or do what I do sometimes and make the first tic of the projectile animation be invisible -- this usually results in making the projectile look like it comes from the end of the gun rather than from the player's chest. It delays the shot slightly, but not enough that it's noticeable.
If you do go with the Neural method, though, keep in mind that you can still make the dummy projectile have a speed greater than zero and destroy it in less than a tic. It's easy enough to make it never affect actors anyway (+THRUACTORS mainly) even if it lasts for a tic, so no biggie there.