Most popular 7mm round, with second set of numbers, in US?

The second set of numbers represents the length of the brass in millimeters. A 7x ("x" reads "by") 57 means that the case, just like an 8x57 is just that, 57mm long. While strange by US standards, try explaining the meaning of 7-08Rem or 7-30Waters to those on the east side of the pond. I also have and love my 7x57. Very mild, but shoots good and flat out a lot further than I can see. My neighbor has a Mauser in 7x64. Wonderful round! Finding ammo is a bit problematic though. He forms his brass from .270Win Seems to work for him.

The second number usually identifies the case length of the cartridge. The one exception I can think of is the .7mm08 which is a .308 Winchester necked down to .7mm (.28 caliber). In Germany the most popular .7mm is probably the old .7mmX57 Mauser. The 7.62X51 is the NATO cartridge, also known as the .308 Winchester. The 7.62X39 is the Soviet or AK-47 round.

The most popular .7mm Stateside is probably the .7mm Remington Magnum. Now there also is a .7mm Remington Ultra Magnum and a whole bunch of 'short and ultra short' magnums. Get the Remington mag if you can find it, or the 7.62X51 (.308 Winchester). The 7X57 Mauser is a good, mild shooting deer caliber. The second number, in this case the '57,' refers to case length.

H

The 7 X 57 Mauser is very common over here. I have one myself. If you are buying used, the thing to look for is barrel corrosion and stretching of the receiver. Go to a range and fire a few rounds, look at the shell casing after firing, look for powder burns around the outside tip of the casing where the bullet seats. You might also look for serial numbers. They numbered different parts of the weapons with the serial number, non-matching numbers indicates someone salvaged parts and assembled a "new" rifle. This may or may not be good. In any event, take it to the range and see how it shoots!

Ditto what H said, I have 7mm remington magnum also, very nice shooting rifle.... deer hate it!!!