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The hop up unit is a separate unit from the gear box acting as an interface between the firing system on gas guns or gearbox on an AEG and the inner barrel. On most AEGs the magazine will feed directly into the hop up unit. The hop up is used to direct the round down the barrel while applying backspin (“hop”) to increase range. The main components are:
A hop up rubber seats at the back end of the inner barrel inside of the hop up assembly. It prevents BBs from double feeding, provides an air seal and applies the required resistance to effect backspin. This is performed through the usage of a protruding mound which makes contact with the BB to apply “hop”.
*Note* Over time rubber will wear leading to performance issues.
The hop up nub sits at the end of the output of the adjuster assembly and acts as an interface between the adjuster and the hop up rubber. Stock nubs are typically a soft piece of tube which allows for flexibility once the BB passes through the unit and compresses the assembly. Wear on the nub will cause uneven compression leading to inconsistency or poor shot quality.
When a BB passes through the hop up unit and inner barrel the hop up rubber will make contact through a window in the inner barrel. The adjuster assembly is used to put pressure on the hop up rubber so that it contacts the round as it passes through it. This results in backspin similar to that of a Flatline Barrel in paintball which increases the effective range. For example, lesser hop will lead to shorter distance travelled while too much hop will increase distance but hinder accuracy.
Most hop ups are typically made of plastic with higher end guns coming with metal units. The metal housing suffers from less stress when a round travels through it; this will allow for a more consistent shot. The stock green G&G rubber is widely regarded as one of the best performing, especially in cold weather.
Here are some common issues that occur in hop up units and how to resolve them:
Issue – Gun is double feeding
Cause – Rear edge of hop up may be worn and therefore will not provide resistance. Remove to confirm.
Solution – Replace hop up rubber
Issue – Gun is not feeding and rounds are dropping out of the hop up unit when magazine is removed
Cause – Hop up is over adjusted and blocking the barrel
Solution – Properly adjust hop up
Issue – Poor range/ low hop, adjustment does not effect performance
Cause – Hop up nub damaged or missing
Solution – Disassemble and inspect hop up, replace is required
*Note* these solutions will not apply in all situations but are viewed as the most common issues and causes.