These tighter diameter barrels have a machined chamber specifically for o-ring pushers in the Caliburn (40cm and 45cm Chambered) and SPAMF (10cm Chambered). There are also unchambered versions specifically for the Lynx by Orion Blasters, though they should also work with other blasters that have pushers that do not use o-rings.
Description from Kuryaka: "The inner diameter of 13mm (0.510"), is more airtight than the 0.527" barrel used in the Takedown Mag kit and basic Caliburn builds without being as tight as 17/32 brass. It will only work with narrow-headed darts as a result, but results in better efficiency.
Normal tight barrels will not work with Caliburn-style o-ring pushers without modification. These have a 10mm "chamber" at 13.5mm (0.53") ID on one end, which fixes that problem."