A method of making a medical barrel for a medical cartridge or syringe, the method comprising: • providing a medical barrel comprising a wall having a generally cylindrical inner surface defining at least a portion of a lumen, the generally cylindrical inner surface having a diameter in the range from 4 to 15 mm; • providing an inner electrode having an outer surface including a portion located within the lumen and coaxial with and radially spaced from 1.2 to 6.9 mm. from the generally cylindrical inner surface, the inner electrode having an internal passage having at least one outlet; • providing an outer electrode; • providing one or more magnetic field generators (61, 62) between the barrel and the outer electrode; • introducing a gaseous PECVD precursor into the lumen via at least one outlet of the internal passage; • applying electromagnetic energy to the outer electrode at a power level of from 0,1 to 500 Watts to form a plasma enhanced chemical vapor deposition (PECVD) gas barrier coating or layer on at least a portion of the generally cylindrical inner surface, the barrier coating or layer having a mean thickness; and • applying a magnetic field adjacent to the medical barrel while applying the electromagnetic energy, optionally for the entire applying step, under conditions effective to reduce a standard deviation of the mean thickness of the gas barrier coating on the generally cylindrical inner surface wherein the magnetic field is provided by one or more magnetic field generators (61, 62) near and extending axially along the length of the generally cylindrical surface, each magnetic field generator (61, 62) having a north pole and a south pole defining a polar axis, wherein the condition effective to reduce the standard deviation of the mean thickness of the gas barrier coating on the generally cylindrical inner surface is that at least part of the time while providing the magnetic field, one or more of the magnetic field generators (61, 62) have their polar axes generally parallel to the axis of the generally cylindrical surface. • providing a medical barrel comprising a wall having a generally cylindrical inner surface defining at least a portion of a lumen, the generally cylindrical inner surface having a diameter in the range from 4 to 15 mm; • providing an inner electrode having an outer surface including a portion located within the lumen and coaxial with and radially spaced from 1.2 to 6.9 mm. from the generally cylindrical inner surface, the inner electrode having an internal passage having at least one outlet; • providing an outer electrode; • providing one or more magnetic field generators (61, 62) between the barrel and the outer electrode; • introducing a gaseous PECVD precursor into the lumen via at least one outlet of the internal passage; • applying electromagnetic energy to the outer electrode at a power level of from 0,1 to 500 Watts to form a plasma enhanced chemical vapor deposition (PECVD) gas barrier coating or layer on at least a portion of the generally cylindrical inner surface, the barrier coating or layer having a mean thickness; and • applying a magnetic field adjacent to the medical barrel while applying the electromagnetic energy, optionally for the entire applying step, under conditions effective to reduce a standard deviation of the mean thickness of the gas barrier coating on the generally cylindrical inner surface wherein the magnetic field is provided by one or more magnetic field generators (61, 62) near and extending axially along the length of the generally cylindrical surface, each magnetic field generator (61, 62) having a north pole and a south pole defining a polar axis, wherein the condition effective to reduce the standard deviation of the mean thickness of the gas barrier coating on the generally cylindrical inner surface is that at least part of the time while providing the magnetic field, one or more of the magnetic field generators (61, 62) have their polar axes generally parallel to the axis of the generally cylindrical surface. wherein the magnetic field is provided by one or more magnetic field generators (61, 62) near and extending axially along the length of the generally cylindrical surface, each magnetic field generator (61, 62) having a north pole and a south pole defining a polar axis, wherein the condition effective to reduce the standard deviation of the mean thickness of the gas barrier coating on the generally cylindrical inner surface is that at least part of the time while providing the magnetic field, one or more of the magnetic field generators (61, 62) have their polar axes generally parallel to the axis of the generally cylindrical surface.