I think this is the raised area left on the bullet where the grooves have been made by the rifling of the barrel.

When a bullet is fired through a rifled firearm, it is engraved with the characteristic land and groove marks of the barrel. The depth and the nature of the engraving depends upon the condition of the rifling(worn or not), surface irregularities of the barrel, diameter of the bullet, "set-up" of the bullet in the barrel, construction material of the bullet etc. Some of the deeper engravings are likely to leave an impression on the under surface of the jacket and on the portion of the lead core which is in close contact with the under surface of the jacket. Marks on the upper jacket surface due to impact and rubbing are not expected to have a significant influence on the under surface of the jacket and on the lead core unless they also come in the direct contact with the striking objects, or unless the engravings due to rubbing and impact on the upper jacket are deeper than the engravings due to rifling. These marks whenever present can therefore be of great significance where the normal identification procedure is not possible.

Uit onderstaande tekst maak ik op dat 'grooves' de groeven zijn en 'lands' de ribbels.

Each barrel has different lands and grooves. The "lands" are the raised parts inside the barrel, and the "grooves" are the recessed portion. The number of lands and grooves, their size and shape, may assist in determining the make and the type of weapon that was used.
http://www.virtualmuseum.ca/Exhibitions/Myst/en/rcmp/firearm...




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Note added at 54 mins (2004-04-16 07:24:18 GMT)
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B 24 B 3/24 . van boren (door de schacht te voorzien van groeven of ribbels B24B 19/04)
http://www.ipc-nl.nl/ipc_b_tekst/b24b.htm

In militaire dienst spraken we over trekken en groeven