Apologies, but really need help.

[Nawalany]

I've googled all of the interwebz and joined about 3 different forums (including this one) to find the answer to my simple question:

How do I load a grenade launcher (underslung or otherwise) or an RPG?

I've made sure the ammo is correct for the weapon by checking the info panel.

I've tried dragging and dropping.

I've hit A for auto reload.

I've put the ammo in various places, the RPG in various places.

I can't load the damn thing. Please help.