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. 
 
If your gun is not listed in the compatibility database in our Help Center, a recommended nozzle length may calculated by measuring your original gearbox and nozzle. Using a set of calipers and following the diagram below, measure these specific areas of the gearbox and input them into the following formula. Please note that measurements need to be in thousands of an inch (0.001").
 
A more precise measurement (which accounts for the hopup and bucking in use) may be obtained by installing a nozzle into the system which is slightly too long for the replica and measuring the nozzle setback when placed into the hopup.

The measured tip length should be within +/- 0.010" of an exiting nozzle to be compatible. If the tip length you measure does not match up to a production nozzle within +/- 0.010" then a nozzle is not currently made for that rifle. However, please contact us with your measurements as custom nozzles can be special ordered.