The 6.5x53R aka .256 Mannlicher headspaces on the rim, not on the case shoulder like rimless cartridges. You may use patches of tape on the head to check. If a case with two layers of tape does not allow the bolt to close, headspace is ok. The chambers may be bored a bit deep. This was done intentionally to allow for oversized/dented cases or some dirt. Factory spec cases will simply fireform on first firing. But when reloading such fireformed cases, adjust the sizing die so it does not push the new shoulder back to factory standards. This prevents case stretching, case head separations and prolongs case life substantially. When making 6.5x53R cases from .303 brass, you may have to reduce the rim diameter slightly. Some individual rifles take the .303 rim as is, others don’t, but the reduced rims feed better through the available Dutch and Portuguese clips.