Order the upgrade and cancel it and then spam the order quickly, you can do this with manually but i recommend using a autokeyboard such as this http://www.murgee.com/auto-keyboard/
you may have to give it a few tries but these are the results:


So far it works on rifle flamers, obers and grens. You still need the weapon slots to be free and you need munitions for the second one aswell.
hf guys

