I have two mint original M98 Peruvian barreled actions in 7x57, both have 29 inch barrels. So, it appears to me the Mauser Factory decided upon 29 inch barrels for their Military Rifles in 7x57 sold to Peru.

However, my hunting buddy has a 7x57 with a 24 inch barrel and it performs very well.

Go with what VonGruff says and you will be happy.