Please find the accurate measurement for your specific airsoft replica before purchasing. Consult the chart below to find which #number you need. 

From PolarStar:

'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 (excluding some model specific Kythera systems). So, if you determine your replica takes a #11 nozzle, then you will need to install the #11 nozzle for whichever system you are converting your replica with.

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 hop-up 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 hop-up.