Hammer forged 6-right
polygonal rifling pattern
Courtesy of FirearmsID.com
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Polygonal rifling prohibits the examiner from measuring the width of land and groove impressions because the lands and grooves have a rounded profile instead of a well-defined rectangular profile.
Polygonal rifling is most frequently observed in firearms marketed by Glock and Heckler & Koch. In the FBI GRC file, the land and groove widths for these firearms are listed as 0.000.
Firing a cartridge in an inappropriate firearm with a barrel of a larger diameter than the bullet may leave no rifling impressions on the bearing surface of a bullet.
Courtesy of Jack Dillon
An adapter (subcaliber device) is an auxiliary chamber inserted into the chamber of a firearm, allowing the firing of smaller diameter or lower power ammunition. Adapters may or may not be rifled. If rifled, the impressions would originate from the adapter, not the firearm. Several adapters are listed in the GRC file.