Rimless
A cartridge case whose rim is flush with the body and which headspaces on the shoulder, the standard design for modern bolt-action rifles.
A rimless case has a rim cut to the same diameter as the case body, with an extractor groove machined just ahead of it so the extractor still has something to grab. Because the rim does not protrude, these cartridges feed smoothly from a box magazine and stack without the rim interlock that rimmed rounds have to manage. The great majority of precision rifle cartridges, from the .223 Remington to the 6.5 Creedmoor, are rimless.
Without a protruding rim to stop forward travel, a rimless bottleneck cartridge sets its headspace on the case shoulder. That shoulder datum is what gives consistent, repeatable positioning in the chamber, which is one reason the design dominates accuracy-focused shooting. The main alternatives are rimmed, semi-rimmed, and belted magnum cases.