A couple of other ways to fill these cartridges rather than the two-needle method:
1) Hold the cartridge down firmly onto a flat surface so that the bung cannot be forced out by overpressuring the cartridge. Inject as much liquid as possible (will be around 1mL), then vent cartridge by first assuring that fill needle is in air space above liquid and then pull out and remove plunger from syringe. If you have a syringe style that does not allow removal of the plunger, then you can carefully detach the needle from the syringe (make sure you draw a little air into the syringe first or it will spew). Repeat until cartridge is filled.
2) Insert your fill needle without syringe attached (or with plunger removed). Use the insulin pen or a syringe plunger to push the bung into the cartridge all the way. Now attach your filled syringe to the fill needle and fill the cartridge. Bung will easily slide down as cartridge is filled. If you don't have detachable needle fill syringe, then you will have to remove syringe, re-insert plunger, fill syringe, then re-insert into cartridge and fill.
Some will say that method #1 is less sanitary than two-needle method and that method #2 exposes inside wall of cartridge to contamination. Both true. However, I don't believe that any of these cartridges were ever sterilized in the first place. Based on what I've read, I believe that these components are shipped from the manufacturer as "Washed, Ready for Sterilization." Everything I've read indicates that pharmaceutical regs require the end user/packager to be responsible for sterilization as sterilization must occur as closely as possible in time and space to preparation of final product. I think that these sellers on AliExpress are just assembling the components (glass, bung, septa, cap) and slapping them into a sterilization pouch (which, BTW, has NOT been through a sterilizer) to make things look copacetic.