Another thing which comes to mind is that you may need more than one resource for "merging". Think of making a cartridge, you need primer, case, powder, and the bullet. However for pre 1900 you may need to make a paper cartridge, where you need paper, powder, wad, shot.

Also some "merges" may need nothing, which will be solved by the button e.g. you seperate something into parts e.g. pull the retaining pin will break up an AR into an upper and a lower receiver (and also may drop the bolt carrier group on the floor).

NAS has a weird thing I discovered, you can right click on an attachment slot and it will bring up a pop up menu. It looks like if you have one of the matching attachments in your inventory, you can select it from the pop up menu which will remove the attachment from your inventory and add it to the attachment slot.