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 Tokyo Marui VSR-10 (430mm)
Internal diameter 6.01mm built for precision shooting
Suitable for Tokyo Marui VSR-10 Series Airsoft Electric Gun AEG Rifle
Genuine Products by PDI
Stainless Steel Tightbore Upgrade Barrel
Advance Technology forming by Cold Forging on Stainless Steel result in tolerance no more than