6.01mm inner barrels are the tightest bore barrels available for airsoft. Precision machining is crucial for these barrels to provide the best possible trajectory. Likewise precision BBs must be used when a 6.01mm barrel is installed in your gun. The tighter bore on the barrel increases the likely hood of a jam if poor BBs are used or if the barrel is not maintained. Tacticalstore only recommends 6.01mm barrels to experienced airsoft players