Meh - we did this for our launchers - but it lead to problems of being kicked when players sold ammo - which they ALWAYS do!!
We:
made the launchers come with as many ammo as needed (1 in your case - 50 in ours
).
Set the launchers price - and to sell at a station.
Set the AMMO to sell at same station (otherwise you CANNOT get ammo with the launcher!! It NEEDS to have the station selling the ammo!
Now - in weapon_goods.ini file - make the PRICE of the AMMO to 0
Yep - nothing. Why? Cause you cannot buy it for nothing - so it doesn't even appear in the list! Ergo - they can get a launcher with one round already attached (which is very like what you want) - and thats it. To reload - they have to sell, and rebuy the launcher.
There is one problem - if this is in multiplayer - they cannot then sell the ammo - of course! Oh well