Being a resident of North America, I have no idea if either of these would be effective against a Hippo, but they're currently the most powerful weapons I've got, so... Better than nothing, I hope...
This is a Heckler & Koch USP45 Elite, is essentially the longslide target shooting variant of H&K's USP (Universal Self-Loading Pistol) chambered in .45 ACP.
Yeah, I know what you're thinking; ".45 ACP? Against a Rhino?! Are you nuts?" Well, yes... However, there's more to this than meets the eye. You see, the USP45 is the offspring of the old USSOCCOM Mk23 Mod. 0, which was a mammoth .45 ACP pistol designed to shoot overpressure loads for use as a primary weapon for SpecOps who needed something smaller and lighter than a Rifle, yet still packed plenty of power.
Long story short, this pistol can safely and readily fire overpressure variants of the .45 ACP cartridge such as .45 Super and .45 SMC, which are ballistically similar to a full-power .357 Magnum or 10mm Auto load, but with .452dia, 230gr projectile. So yeah, it's quite a little hotrod, really. Hot enough for Hippo? I dunno, but it's the most powerful handgun that I own.
This is a Mossberg 590 Shockwave chambered for 12 Gauge 3" Shotshells, pictured with a box of Brenneke Magnum Slugs, nuff said. Can a 12 Gauge Brenneke Magnum Slug stop a Hippo? I don't know, but I'd be very surprised if they could shrug off taking one of them to the skull. I mean, it's a 600gr Slug traveling at 1500fps with 3000ft-lbs of energy at the muzzle, 1200ft-lbs at 100 yards. I've heard that poachers have used similar slugs to drop Elephants, but I have no idea if that's true.
If not, then don't hesitate to let me know, as I'd love even the most absurd of excuses for why I might need to own a more powerful firearm.
An excuse to buy a new Taurus Raging Hunter .460 Magnum and a Marlin Model 1895 SBL .45-70 would be much appreciated.