I've killed a fair number of bulls and have used .284 bullets for two of them. I once used a 120 TSX and its performance was impressive. I used a 162 A-Max and it too performed beautifully. The 7x57 will be plenty of cartridge and there's little need to overthink the bullet. I'd be using a 120-140 grain monolithic (TSX, GMX, etc.) or a 150-160 jacketed bullet.
