I pointed out the website in a post last week, but I would suggest reading articles on feeds on the website devoted to
Fountain Pen Design. You may find it to be useful.
The feed and breather tube shown in the OP are for a Parker Vacumatic. The breather tube is designed to help get ink
into the pen, not from the ink reservoir. Vacumatics fill incrementally, up to the top of the breather tube. The same for a Waterman Inkview, and for many Eversharp Skylines. The end of the feed has to be exposed directly to the ink for the feed to work in the pen. The breather tube connects to a hole that comes up under the feed. With a wide open tube like that, it's capacity to carry ink to the nib is minimal.
Cartridge/converter pens have an insert or extension that goes from the end of the feed through the breather tube into the cartridge or converter. This has one or two very fine slits to carry the ink, and a wider space at the top to allow ink back into the pen. The metal or plastic tube that is on the barrel end of a nib assembly is the piercing tube, there to punch the hole in the cartridge, or dislodge a plug. The actual work of carrying the ink through the tube to the rest of the feed and ultimately to the nib is done by the insert (on Sheaffer HR feeds) or the extension/tail on plastic feeds.
The paperclip will rust. If you want to keep using that feed, look for a vintage Sheaffer feed with a hard rubber insert. They were used in many pens, including the Imperials and early NN pens. You may be able to get it to fit into the end of the Vacumatic feed that you are using. IF it works, you'll find that ink flow is much more consistent and better regulated. Sheaffer drilled their feeds nearly to the front end of the feed, and the ink flow was conducted up through a slit in the top of the feed that ran almost the entire length.
This image shows a Sheaffer Imperial feed - the insert sticking out to the right pulls out. Others in earlier feeds found in the Balance and 50s open nib pens are a bit fatter. Note the flat top to the insert which allows air back into the pen, and the narrow slit to carry the ink.
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