You could try something like this:
Put this in the tank's init field:
this removemagazines "shell73"; this removemagazines "heat73"; this removemagazines "gun73";
Not too sure if this will work, I'm just throwing a suggestion out there. Also, if you look in the comref, you will notice several different calibers? of shell, heat, and gun:
Shell73
Heat73
Gun73
Shell105
Heat105
Gun105
Shell120
Heat120
Gun120
Shell125
Heat125
Gun125
You may have te experiment with one of these values to eliminate the shells.