the "weaponname" depends on the type of the tank.
Generally, there are shells and heats, but with various numbers.
Abrams - 120
M60 - 105
T-80 - 125
T-72 - 125
BMP - 73
T-55 - 105
if you want, for example, an Abrams to load a HEAT, you write nameoftank selectweapon "HEAT120"
if you order him to load for example HEAT 125, I suppose it won`t work.