I had to do some poking myself because this gun intrigued me. Incidentally, I've owned a similar Isaac Hollis in 500 Nitro-For-Black and I can say that the quality of the guns are typically pretty good for that maker at that time period.
Here are the things that I wasn't so crazy about:
The 1800FPS with a .577 2-3/4" case is based on old Kynoch data. That data was using a 28" barrel, and on top of that they exaggerated the performance. If this is a 25" or 26" barrel, in all likelihood the gun will regulate at a speed of around 1625-1675 FPS. So if indeed the barrel is shorter than 28", there went velocity. Even with the 28" barrel, it will probably regulate at 1750fps because the data "embellished" a bit.
Next issue. A .577 is a stopping gun. Stopping guns need solids. A pre-WWI double rifle cannot use a modern solid. The modern mono-metal solids are too light by volume, aka they are too long. So first problem is that a modern solid will literally separate the ribs because a 650-750gr .577" piece of brass/copper doesn't turn corners very well and the barrels are not straight. Add to that, the original "solids" of days gone by do not exist anymore. The "original" solids were soft lead with a steel jacket, a meplat (hopefully) to prevent the lead from oozing out the back of the bullet, and then were covered in a copper jacket. Those don't exist anymore. As of last I time I checked, not one soul on the planet was able to make them. Today, cup-and-core bullets are made ass-end to tip, whereas the "original" solids were from bullet tip to the butt. The butt would have a hole in it and the meplate would mitigate lead squirting out of the solid upon impact. So no original solids exist and modern solids won't work on this gun.
Next issue. The .577 bullets that exist will not open correctly at such low velocities. Yes, I'm sure Hawk can make you some soft bullets that might open at lower velocities, but at that point they are so lethargic due to the low velocity from Point #1 above that the penetration is going to be less than desirable.
Next issue. You don't have pin gauges yet to run through the bores to figure out what the bore diameter is all along the rifle. IF the bore diameter isn't standard, then you're in a further pickle trying to find / make bullets to feed the thing. Buying swaging dies isn't an easy solution because what would you swage against, some proprietary never-to-be-made-exactly-the-same run of Hawk bullets? If Hawk goes belly up, what then? (Woodleigh, Peregrine, Cutting Edge, North Fork are no-go in this scenario, as is patched lead)
Some rough data to consider as well:
750gr bullet at 1750fps out of an 11lb gun is giving you 89.5lbs of felt recoil. That's a hell of a punishment on the shoulder, a full 20 pounds more than a 470 Nitro Express and 15 pounds more than a 500 Nitro Express. Yet the bullet selection is problematic, the velocity is problematic, and there is no solid in existence to give you the superior penetration that is required to "make the gun work as originally intended".
750gr bullet at 1750fps gives you 5099 energy, 187 momentum, and 108 Taylor KO rating.
At 35 yards using 750gr and 1750fps you actually have around 4702 FPE and at 50 yards you're at 4540 FPE. (Penetration issues and bad bullet options get extra dicey here)
Now suppose my fears about regulation in a shorter barrel pan out. If that's a 26" barrel and it regulates at 1675fps, this is what you're dealing with: 4673 FPE at the muzzle. 4305FPE at 35 yards. 4156FPE at 50 yards. At the muzzle 179 momentum and 103 Taylor KO rating...but fading fast.
If this was your 5th double rifle in your stable, I'd be rooting for you to buy the thing because it just cool. But if this is your first dangerous game rifle and if you actually are thinking of hunting dangerous game in Africa, this is not the gun for that job. From nasty recoil to lethargic velocity, horrible bullet selections in softs, no options to use solids, and for many more reasons, this is not the gun to bring down an elephant and may prove more marginal than you'd think on Buffalo.
Some errata to bolster my claims:
Rifleshootermag.com article mentions problems with velocity even with modern .577 3" ammo as follows: "One of the things we learned was that much of the .577 ammo out there is loaded too slow. The stuff we had fail in 2008 chronographed at about 1,850 fps, 200 fps below specs. Recoil wasn't pleasant but was sustainable--except, with the kind of frontal area presented by an elephant, we didn't have the velocity we needed to ensure penetration." <- Reason, the kynoch data from 1926 was A.) Lying, and B.) Using 28" test barrels. In the real world, even the .577 3" has some concerns, the 2-3/4" much more so.
