Fountain Pens

Interesting, I'd never heard that one before. I'm familiar with the pens but not the converters. Unfortunately, the converters were probably chosen just for the dimensions and aren't actually made by Hongdian. It would probably take some trial and error to find one that works better than the one you have. From a quick search they look like a few I have for some other Chinese brands.

My 2 cents is that either a) that small amount of empty space isn't really going to make a difference in the long run. Fill it, write with it, have fun, enjoy the pen.

Or b) if it does bother you, a blunt-tipped syringe could be used to fill the converter instead. In some cases I will just fill the converter directly from the bottle/sample then plug it into the pen. It's a lot cleaner but you'll have to wait for the ink to flow to the tip. But there is potentially more ink in the flooded feed than the small amount lost by the odd converter fit. So to get a truly "full fill", you'd need to fill as usual then take the converter off the rest. But that sounds messier than its really worth in my opinion.

If you're feeling handy and it really bother's you, c) It might be an assembly error. You might be able to disassemble the converter and setup the piston so it doesn't press on the feed. On some converters you can uncrew the upper silver collar and pull the whole piston out. This would be a last resort since it could mess up the converter completely. For context of a similar situation, I was cleaning my TWSBI 580 and removed the piston to really clean out the barrel. When I put it all back together the piston wouldn't go back all the way. The piston needed to be set at the proper length before tightening it all down. I have a feeling it was something similar from the factory. Unfortunately, quality control is the first to go on these cheaper pens.