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 Inner Barrel for Marui M4 Short/ MC51 (285mm)
Internal diameter 6.01mm built for precision shooting
Suitable for Tokyo Marui M4 Short/ MC51 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