The reason it doesn't work, isn't a design flaw. It's because it looks like the gun and plunger are allready combined when you collect them (the way it's drawn in the inventory). So combine them (even if you think the are allready combined.
This should solve this prolem.
Thanks Seffen99, you make a good point!
The gun and plunger are quite obviously seen as separate items in the inventory
when playing on a full-sized computer screen but they become less recognizable as separate items as your screen size is diminished for iPADs, Androids, etc.
Just "combine them even if you think they are already combined" and you'll be fine. :~)