The hammer at the top of the Rhino is not an actual hammer.

I realise this. Doesn't change the fact that its possible the handcannons in Destiny work the same way it does. Although more than likely the guns are energy based and the hammers and general style are just asthetics, while the actual mechanics are closer to just an energy based weapon or possibly they behave similar to Mass Effect weapons do.

Well Mass Effect was explained as all weapons using alloy blocks which the weapon "shaves off" tiny projectiles and hurls them at an incredible velocity. They explained it as the weapons having a nearly limitless supply of ammunition, with the only issue being heat dispersion... which never made sense to me. If you're removing material from a block, you're eventually going to run out of that block... no matter how miniscule the amount. I don't think Destiny works that way, or reloading wouldn't really be a thing. Even in Mass Effect 2 and 3, you're not reloading your weapons, you're just expending overheated thermal sinks. Which again, doesn't really make sense... but I was able to suspend my disbelief for those games. MY problem with Destiny is that while futuristic, they ARE still based on modern firearms in many cases. I mean hell, the shotguns are even pump-action...

They do mention guns in Mass Effect do run out of actual ammunition and need replaced after some time. The standard M-8 Avenger ammuntion block supposedly lasted around 50,000 rounds. As for heat sinks, they were essentially just cylinders made of heat conducting materials that the heat was directed torwards. Ejecting one and putting a new one in simply allowed the gun to continue to fire without damage. A gun could continue to fire if not using one or one that was already used, but the heat would quickly start to melt the barrel and other components. As far as Destiny is concerned, handcannons (and some others) are likely energy based weapons. The Thorn is a prime example. Its rounds are as large as the gun itself.

Yeah, which is why it made little sense when they implemented heat clip ejecting and completely removed overheating. They should have made it so that the clips could be ejected for longer sustained fire, but if you had no clips, the gun would overheat and take much longer than an ejection to cool down again.

They explained the change to heat sinks lore wise as using heat sinks allowed a soldier to fire more rounds down range in a shorter amount of time compared to an advanded heat dissapation system. The trade off being you had to carry around heat sinks. Chances are they never implemented the ability to just wait for it to cool after running out of heat sinks was down to simplify gameplay. In the books and other media, there are several references to people doing this or using older weapons (M-7 Lancer).

Well in theory, an overheated sink will cool eventually. So when you're out of sinks, you could simply not eject the last and wait for it to cool before continuing to fire. I understand it was a change dictated by gameplay, but still... it would have made more sense to include both, even if making the game slightly more complicated from a development standpoint.