Both are really good calibres . The .270 Winchester traditionally uses bullets weighing up to 150 grains . The 7x57 mm Mauser typically uses bullets weighing up to 175 grains .
If you are limited to American ammunition brands only , then I will recommend the .270 Winchester . When I hunted my first mule deer in the United States in 1980 , I used a 7x57 mm Mauser which a friend had lent me ( it was a BRNO ZKK - 600 ) . I was given some Winchester Super X 175 grain cup & core soft nosed factory loads . I was not very impressed with the velocity ( they were OK for the small Texas whitetail deer , however ) . Over the years , I came to realize that American ammunition brands ( such as Winchester , Remington and Federal ) tend to significantly reduce the powder charge in their factory loads for the European calibres ( such as 7x57 mm Mauser and 8x57 mm Mauser ) . However , they always load their American calibre offerings ( such as .270 Winchester ) to their full potential.
But South African PMP / Pro Amm ammunition for the 7x57 mm Mauser is exceptionally good ( especially the 170 grain soft nosed loads ) . It is loaded to the correct velocity and you will be very pleased with it . If you have access to European and South African ammunition brands , then I would strongly recommend the 7x57 mm Mauser .
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