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It’s working as intended!
The Bornheim has a small opening at the bottom through which the empty magazine falls out automatically.
If you have less than 5 rounds left, you cannot load a full clip in. So what do you do? Easy: Put an empty clip in and use your finger to prevent it from falling out. Then, start inserting whatever spare rounds you got left from the top to fill it up.
Like with many of Hunt‘s weapons you can optimize reload times for the Bornheim, if you pay attention to the weapons‘ mechanical details. In this case, try to reload full clips with five rounds and avoid less than 5 extra for the last reload, if you fully emptied the weapon. Saving those last 1-2 rounds can speed things up a lot!
Guns of the era were very often clunky and highly experimental still, meaning many handling procedures known today, simply were not invented in 1895.
Hope this helps clear this up a bit! :)