For cylinder conversion systems we assign 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 we produce numerous different nozzle lengths to provide compatibility for a wide range of replicas. Below we are listing 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 we have measured and calculated the compatible nozzle length, please visit our 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. 
 
*NOTE: The Kythera models which are made for a specific replica (e.g. A&K/JG SR-25) with a non-standard length cylinder will use a specific nozzle with a "K" prefix. Since they use a non-standard cylinder, the universal nozzle numbering system cannot be used for these models. However, the design and profile of the "K" nozzles are the same as the standard numbered nozzles and are cross compatible between standard and non-standard cylinders.

Additionally, all nozzles are marked with their number with the exception of the #1 nozzle. These are produced in far greater quantities than all of the other nozzles so they are left unmarked to reduce production time.