Is there a trick to getting round HVAC duct pieces to fit together
You need a duct crimper like this to reduce the diameter of the end of one piece of duct:



Run the tool around the end of the duct so that you have multiple parallel crimps. That piece should slide right into the uncrimped piece, giving you a secure connection to tape.

If you don't want to spring for a dedicated tool, I've seen where people use needle-nosed pliers to do the crimping.