If you try to dissolve a large amount of Hg in dilute to moderately strong HNO3 all you will get is COPIOUS amounts of mercurous nitrate. The HNO3 has to be strong if you don't want to deal with this shit ton of mercurous salts totally passivating the mercury. You can tell when this happens because it will break up to a light grayish powder. That's passivated mercury. If you try to add HCl to form aqua regia it won't do anything except add mercurous chloride to the mix. It cannot oxidize Hg to the +2 state when anything but concentrated. If you add elemental bromine nothing will happen either even if you heat the shit out of it. I found that you must add HCl AND Br2 if you want it to work. I think I may know why. If you can have a Br3 ion and a Cl3 ion why not a Br2Cl ion? After all addition of chlorine to HCl forms trichloride and addition of bromine to HBr forms tribromide. Why wouldn't chloride added to Br2 form Br2Cl? That would be a stupidly powerful oxidizer. Whatever is happening the combination of Br2 and HCl will dissolve mercurous salts rapidly when heated. I depassivates the metal to the point that stirring will cause the Hg to clump up into pools of shiny mercury again and the acid can attack it. I would imagine that chlorine gas would work just as well as bromine does.