Precision tight-bore inner barrel for Tokyo Marui Model 18C gas blowback pistol. This stainless steel one-piece barrel boasts an internal diameter of 6.01mm with a margin of error equal too or less than +/- 0.002mm.
Such a drastically tight diameter can potentially afford you great accuracy but the extreme dimensional tolerance means that it is critical you use the appropriate variety of high quality BBs and you regularly clean your barrel as even the slightest grain of debris can create problems when the gap between BB and barrel is so small.
BBs do not actually touch the inner barrel but rely on the stability of riding on a cushion of air pressure as it hovers away from the inner sides of the barrel. A tighter space means a better seal and a straighter pathway but also afford much less tolerance for more irregularly sized BBs, large BBs and even the slightest bit of fouling in the barrel.