But then, if the smoke is indeed from a prototype gun test, wouldn't that mean one would be able to discern the changes/modifications in the airframe in that particular area? That also puts into question the various (supposedly) locations of gun related port by @Inst.Note the smoke near the portside of the aircraft. This is a photo of the J-20 literally shooting down theories that it carries no gun (pun very much intended).
The gun cannon on the J-20 was discussed before. Some engineer said the J-20 has a compartment designed for a cannon. If and when they believe the J-20 needs one, they will install them.