Whatever shoots best and holds together reasonably well. We used 140gr TSX bullets in my boys .284 at just over 3000fps with great success last summer in RSA. You wont be able to quite drive them that fast in the 7x57 but you should be able to get around 2800 or so which would certainly do the job. Funny thing about rifles and bullets. This .284 is a rifle of my late dads that I actually took my first 2 deer with in the 60's in Alaska. It is a rebarreled pre 64 M70 Win Featherweight with 22 inch Douglas bbl. It was always a very accurate rifle with any number of bullets. Dad gave it to my boy to grow up with a few years ago so we started shooting it again. It was all over the place! I found a crack in the web area of the stock, fixed that, seemed a little better but not much. I had remembered it as already being glass bedded but it wasn't, so I did that and floated the barrel. Got a little better. Than had a local smith recrown it, that didn't go very well but it seemed a little better still. Then took it to a world class smith who laughed heartily at the first crown job and re did it to perfection. Still wasn't shooting like I remembered it would. Gave up on all the standby bullets and tried a new powder and bullet combo. Win 760 and Barnes 140gr Triple Shocks. A bright light shown down thru the clouds from the Heavens and I could hear trumpets and angels in song! Not really just being dramatic! But truth is, it just came alive. Almost any charge would shoot super. I finally settled on 55.5 grs of 760 with that bullet and it goes just over 3k and shoots under an inch at 100 and is lights out on game if properly hit, even wildebeest, which are notoriously tough. New life for Dads favorite old rifle in my boys hands, that got me my first deer. Doesn't get much better than that.