PolarStar F2 #4 Nozzle SR-25, G&G

For cylinder conversion systems Polarstar assigns numbers to the nozzles instead of using the rifle model. This is because the nozzle length can vary between the same gun model when made by different manufacturers. This can be from the manufacturer designing the rifle differently, or from the thickness of the gearbox shell affecting how far from the hop-up chamber the cylinder sits. Therefore one nozzle length may work in different rifle models but not necessarily work in the same model from a different manufacture. For example, a standard MP5 could require any of 4 different nozzle lengths depending on the manufacturer.

For this reason Polarstar produces numerous different nozzle lengths to provide compatibility for a wide range of replicas. Below are listed the make and model of the replicas which the nozzle length is most commonly used with but it may be compatible with more models. For a list of replicas which Polarstar have already measured and calculated the compatible nozzle length, please visit Polarstar's Help Center.

Once you determine what number nozzle your replica requires, that number is universal across all engines* that use the numbered nozzle system. So, if you determine your replica takes a #11 JACK nozzle, then it will also take a #11 Kythera or F2 nozzle if you ever put a different system into it. 

Please note this is not an exhaustive list, take the measurements into account when looking for a model not present in this list.