PDI barrels are cold-hammer forged from stainless steel, which is the same process how real rifle barrels are produced. They have a superior tolerance and roundness of the inner surface compared to anything else on the market.

PDI 01 6.01mm Precision Barrel for Marui Hi-Capa 5.1 Airsoft GBB (138mm)
Stainless Steel Tightbore Upgrade Barrel
Internal diameter 6.01mm built for precision shooting
Genuine Products by PDI
Advance Technology forming by Cold Forging on Stainless Steel result in tolerance no more than
Suitable For Tokyo Marui Hi-Capa 5.1 Series Airsoft Gas Blow Back GBB Pistol