I am legit curious if we have handguns capable of firing 50 cal, or the equivalent, rounds.

The IMI Desert Eagle Mark XIX, Smith & Wesson Model 500, and a number of other handguns say hello.

5951525 Sweet, always been a revolver kind of guy, in regards to the Model 500

We sure do. They're stupid, be we have them.
Here's the problem, at that size of a round, you're dealing with a huge amount of force being used to propel it, which, due to the laws of physics, means you're going to have just as much force pushing back. This pushing back force is called recoil. Recoil will: (1) throw the gun away from where you're aiming it, meaning you have to take time to re-aim the gun, or (2) it will put that force into your wrist, or a combination of the two. This is further made a problem with smaller guns because there's less gun to prevent it from being forced away/absorb the recoiling force. There's also one more thing (and if you're writing a story, I want you to keep this in mind) ammunition is HEAVY. It is oh so very heavy. Plus it takes up space. A smaller round, like a 9mm means that you can have more rounds in a smaller space.
In short, we have 'em, but we don't like 'em (unless you're trying to overcompensate for a lack of rounds and size elsewhere... and if that's you're problem, you're not in the military and for a good reason).