My choices.

I have only ever shot "buffalo" with my .375 H&H Magnum, my 9.3x74R double rifle and my .450 No.2 Nitro Express double.

My first "big bore" was a .577/450 Martini Henry sporting rifle by Rozier. Never shot anything with it I'm afraid. One day ...

My first real sporting big bore was my Whitworth Mauser 98 in .375 H&H Magnum. Used in 2002 to hunt my one and only cape buffalo. Two shots, the second because the bull appeared to be "improving". An exciting hunt not because of the buffalo, the hunting and fun was excellent, but exciting due to the human "interaction" in Zimbabwe. Have hunted cape buffalo since then, but never took one that trip unfortunately. Shot some other bigger beasties with it such as scrub bulls, plus hard to put down donkeys, and endless pigs, and had it along with some water buffalo hunts, but never shot a WB with it. However our friend JB (larcher) did use it to shoot his water buffalo in 2015.

Generally I like hunting water buffalo with my Jeffery double in .450 No.2 Nitro Express. ALL of the big bulls I have shot have been with this rifle. And 480 gr Woodleigh RNSPs and FMJs when necessary. Plus a couple of elephant. It works and provides confidence. Scrub bulls fall to it no problem as well.

My 9.3x74T U/O Tikka double rifle went on the buffalo cull around 2005 or 2006. Worked well, is an ejector, so quick reloading when shooting multiple buffalo in a herd, when one starts to get back up, hit it again. With a scope also allowed brain shots. But learned to never take a brain shot with the second barrel when the first is empty ... because you can miss the brain. Then a chest shot is far better and usually allows time to reload. Scrub bulls, donkeys, horses, pigs.

I will no doubt use my Jeffery again and again. Next hunt I do hope to try out the .404 Jeffery in a Mauser M03 on water buffalo. Scoped and accurate. I think it will provide the ncessary goods quite adequately.

10-bore side by side Westley Richards SHOULD be the next choice. Been too slack to organise it, if the gods smiled it could be ready but very unlikely. An excuse for a future hunt just to use it.

A .45/70 is in the safe, a Baikal scrubber. But will probably not use on buffalo. Would be fun on pigs and assorted other game though.

A .58 Pdersoli BP ML? Hmmmm, it would need to shoot far far better before it was tried. Might not hit the side of a buffalo! :O

Got another one in the safe. Haven't ever mentioned that one yet.

Does have two barrels, hammers, and burns BP in brass cases.