RE the 7x57 Mauser...since Nilgi are very spooky and elusive..ie very hard to get close to at say under 200 yards, I'd offer the only reason a 7x57 would not be my 1st or even 2nd choice would be the slow MV in it's heaveist avaliable bullets.
"Karamojo" Bell killed more elephants that can weigh more than a pile of nilgi once upon a time, than are in all the zoo's in Texas with a 7x57 shooting 175 gr FMJ solids...but at a range reportedly always under 20 yards and closer to 5-10 yards was his preference during a charge. Bell's "sweet spot" was a thin very small gap in the skull that allowed the elephant's brain to be hit, but only from a close & upward angled shot placement.
While I haven't looked at any of the manuals or fired any of those bullets I'd suspect Bell's 7x57 bullet would not exceed 2400fps MV's...and have a substantial 200 yard drop...much less the longer ranges I keep hearing about that nilgi are killed at. FWIW there are a few nilgi killed every year by hunters using archery tackle...aka 7 Mauser ranges...so what do I know???
If you are dead set on using a 7mm of some sort, I'd think a 7STW - 28 Nosler would be a much better choice in mono metal 160-175 bullets and try and break some major bones to anchor the critter.
JMHO & YMMV
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