I used a Heym Jumbo .600 Nitro Express sidelock ejector to hunt a hippopotamus on land (you can see the photo on these forums). One barrel was loaded with a LFB (Labor Fur Ballistik) 900Gr copper jacketed soft point. The other barrel was loaded with a copper jacketed FMJ solid of the same brand and weight. Velocity was 1985fps. Absolutely devastating terminal effect. Rifle weighed a handy 13.5 LB and had 26” barrels.
I now own an August Schuler clamshell action .600 Nitro Express boxlock ejector and am getting ready to take it out on my next African Safari. One of my future challenges will be getting a 900Gr Cutting Edge Bullets Monolithic Solid and an 825Gr Cutting Edge Bullets Raptor to regulate to the same point of impact from my .600. I expect the difference in the two bullet weights to pose a challenge in regulation. If I don’t succeed, then I’ll just use the LFB factory loaded product.
I highly recommend this caliber (bearing in mind that it’s not for everyone), although the recoil does take some practice to counteract properly. But I must stipulate that the velocity of your ammunition should be between 1950-2000fps. And the bullets must be premium grade.
I don‘t consider the .700 Nitro Express to be anything more than a mere toy. A novelty. It achieves very poor penetration on frontal brain shots on elephant bulls.
I would hesitate to use an 8 bore or 4 bore black powder rifle on an elephant. In my experience, they charge the smoke produced by the black powder.