Cylinders manufactured outside the USA may be transported under a special permit , and these have been issued for solid metal and composite cylinders with working pressures of up to 300 bar ( 4400 psi ) by several manufacturers .
Cylinders manufactured outside the USA may be transported under a Special Permit ( exemption ) issued by the Pipeline and Hazardous Materials Safety Administration . 
 Special permits have been issued for solid metal and composite cylinders with working pressures of 300 bar ( 4400 psi ) by several manufacturers .