Fountain Pens

I inked up a Kaweco sport today.

Initially I took a half used Kaweco cartridge out of a Kaweco Perkeo, but it was so loose fitting, I was scared to use it in the Sport.

So I tried a brand new Kaweco cartridge. It fits better, but boy does it feel precarious. The converter seems to hold in place a little better, but not by much.

A search online suggests it's not just me that finds this disconcerting. There are reports of the cartridge detaching when out and about and some are reccommend putting a spring in the barrell to hold the cartridge firmly in place.

Faced with this design floor, I wonder why Kaweco don't improve the fit of the cartridge in Sports series. They've been in the game a long time and should know how to make a better fitment. In fact I know that they know how, because the fitment on the Perkeo is really good. Not as good as some other pens I own (e.g. Sailors), but still leagues ahead of the Sport.

Can anyone here relate?

EDIT: and yes, I know, they released piston fillers recently, but they are quite a bit more expensive.