For barrel conversion systems, we assign numbers to muzzles instead of using the rifle model. This is because muzzle lengths can vary between rifle models from different manufacturers. This may be due to the manufacturer designing the rifle differently or the thickness of the gearbox, which affects the distance the barrel sits from the hop-up chamber. Therefore, a muzzle length may be compatible with different rifle models, but not necessarily with the same model from a different manufacturer. For example, a standard MP5 might require any of four different muzzle lengths, depending on the manufacturer.

For this reason, we manufacture numerous different muzzle lengths to provide compatibility with a wide range of replicas. Below is the make and model of the replicas with which that muzzle length is most commonly used, but it could be compatible with more models. For a list of replicas whose compatible muzzle lengths we have measured and calculated, please visit the Polarstar Help Center.

Once you determine the nozzle number your replica requires, that number is universal for all engines that use the numbered nozzle system (except for some model-specific Kythera systems). Therefore, if you determine that your replica is compatible with a No. 11 nozzle, you will need to install it for the system you are converting it to.

If your weapon is not listed in the Polarstar Help Center compatibility database, you can calculate the recommended nozzle length by measuring the original gearbox and nozzle. Using a caliper and following the diagram in the image section, measure these specific areas of the gearbox and enter them into the formula below. Please note that measurements must be in thousandths of an inch (0.001").

JACK Centerline Nozzles: TIP = F + B - (A/2) - 0.0875

A more accurate measurement (which accounts for hop-up and bucking in use) can be obtained by installing a nozzle in the system slightly longer than the replica and measuring the recoil of the nozzle when placed in the hop-up position.

The measured nozzle length must be within ±0.010" of the existing nozzle to be compatible. If the measured nozzle length does not match a production nozzle within +/- 0.010", then a nozzle is not available for that rifle. However, please contact us with your measurements, as we can order custom nozzles.