I was certified for soldering about 40-45 years ago when I worked in the aircraft industry. Since then I've probably made several hundred thousand if not several million solder connections. The proper way to solder is to first tin the wire. Then fill the solder cup with enough solder so that when you insert the wire the solder comes to the top. The solder should flow up the wire slightly. When you see this, you've got a good connection. To solder 4mm bullet connectors you need a large flat tip. Forget the hole, it's useless. I use a 40W Weller I've had for over 30 years with a 1/4" flat tip. Tin the tip. Hold to the side of the cup and fill with solder. Insert wire and hold until solder flows on it good. Remove iron. Let cool. Done. Flux is not a conductor. If you are using resin core solder, you should not use additional flux.
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