Have not shot a buffalo with that bullet but PHs usually advise against using them for buffalo - soft fragile bullet.
From shooting them into non-living targets, I suspect they have a low velocity limit of around 1800 fps.
Much faster and I would worry about over expansion/breaking up.
The Hornady DGS is a bit stronger and I have shot one (my only one) buffalo and plains game with the DGX at 2050 fps (480 grain) and it worked very well (recovered bullets shown in my photos).
I would not trust the DGX over about that velocity.
If your rifle will shoot the 500 grain Swift A-Frame with reasonable hunting accuracy, that is the soft that I definitely would prefer.
To quote John Luyte of Duke Safaris:
"I've never seen an A-Frame fail".
Thanks Gizmo,
And, my goofy brain missed my own typo on "The Hornady DGS is a bit stronger" and blah, blah, blah.
CORRECTION: "The Hornady DGX is a bit stronger" etc.
Your experiences with the DGX (and DGS as well) are quite noteworthy if not alarming.
Any lead core bullet can easily shatter if impacting heavy resistance at too high of velocity.
The DGX in .458/480 grain and-or 500 grain is probably somewhere below the magic 2125 fps level.
The Swift A-Frame is probably somewhere around the 4,000 fps level.
(And the much lauded Barnes or similar designed Monometal expanding type bullets are probably somewhere around the 10,000 fps level - LOL.)
For my Safari dollar, the only reason to not use the A-Frame as my soft on heavy game like Buffalo or even eland would be if I could not get it to shoot straight in my rifle.
My old double shot so well ("regulated" so well) with the DGX and DGS that I chose them for one safari and they performed very well for me indeed but again, I did not drive them very fast.
My old .450 No2 NE shot very small groups with them at 2050 fps.
For any bolt action or single shot, I'm confident that I probably would be able to figure out a powder/velocity combination that would shoot the Swift accurately enough for hunting (as opposed to competition target shooting, that is a whole different topic).
To whip a dead horse here - the A-Frame is the one I definitely would prefer when a soft is appropriate for heavily built game.
Cheers,
Velo Dog.