I don't particularly care for the 139gr SST's in .284 (7mm). In my 7mm.08 I had some blow up issues at close range impact speeds. Past 70-80 yards they always performed flawlessly and they flew amazingly well but close range shots, on bone hits such as shoulder, they had a tendency to blow up. That 7x57 Mauser is running a good bit slower (with Factory ammo) than I have my loads for in my 7mm-08 so you may not have the same issues. The Accubond and Partition both are good bullets. I've been reloading 50+ years now so my factory ammo knowledge is limited to reports from others pretty much and the little bit that I shoot someone elses rifle with their choice of ammo for testing. Another Factory Ammo choice you might want to give a swing at is the other Hornady Factory offering of the Superformance with the GMX bullet.
Your best bet would be get into reloading! The old 7x57 Mauser is a great old cartridge that is severely underrated and all factory loadings are pretty limited in power. Your M77 can handle a MUCH hotter load than any factory offering you will get.