[Papalote]

I took a bull and a cow with my open sighted guide gun, Remington 405s, and re7. Look up 45-70 Vs nilgai bull on this site. When I was there I witnessed one feller take 5-6 nilgai with a scoped 7mm wsm, he was a top rated clays shooter. The ranch rifles were BAR 300wm. If using a GG, I'd shoot heavy cast or premium jacketed bullets. We killed 17?,iirc, that weekend. Many were taken running, so shot placement was difficult. I had the only levergun in the camp. Few of the bullets passed thru.

Will you be chasing them, hunting from a blind or spot/stalk? A .30-'06 to the neck would work just fine, likely s/s or blind hunting.
The meat is the finest available. Be sure the ranch hands give you all of it. I had to argue to get the carcass, rib cage and flank, that netted 37? pounds of hamburger. I hunted just south of the Falfurrias check point. If I had a 300 WM, I'd use it as I had my bullets fell short at longer distances.

Papalote