Hop-up buckings: shapes, hardness and how to choose the right one
The bucking is the hop-up part that touches the BB and gives it spin. Shape, hardness and compatibility make the difference between a flat shot and BBs that drop or curve. Here are the main variants and how to choose.
Updated 2026-11-08 · 2896 replicas in the database
What the bucking does
It's a rubber sleeve fitted at the start of the barrel. A nub enters the barrel through a window: the BB rubs against it and leaves with backspin. How the whole mechanism works, with an interactive animation, is in the hop-up guide.
orange: contact area with the BB · dark red: the rubber
Shapes
Many makers have their own variants (different inner lips, improved air seals): they almost always start from one of these shapes.
Hardness
| Situation | Indicative hardness |
|---|---|
| Cold weather, pistols and low-power guns | 50°–60° (soft) |
| Mild weather, AEGs and gas rifles under 1 J | 60°–70° (medium) |
| Hot weather, more powerful guns | 70°–80° (hard) |
In winter a softer bucking is recommended: cold hardens the rubber and reduces the effect, so a bucking that works in summer may not be enough. In summer the opposite applies: rubber softens and a too-soft bucking can give too much spin.
Values are indicative: each maker uses its own scale and states on the packaging which power range the bucking is for. Follow those indications.
The nub
The nub is the small piece between the hop-up arm and the bucking. The stock round nub presses on one point; flat or "H" nubs spread pressure over a longer area and are often paired with long-contact buckings. Swapping the nub is cheap and sometimes improves consistency more than a new bucking.
Compatibility
| Gun | Bucking |
|---|---|
| AEGs (rifles, carbines, electric SMGs) | Almost all share the same "AEG" standard: wide choice. |
| GBB and NBB pistols | Specific buckings, different by brand and system: look for ones listed for your model. |
| Bolt-action spring rifles | Depends on the model: many VSR-10 clones accept AEG-type buckings, others use their own. |
| Gas rifles (GBBR) | Depends on the system: some use AEG buckings, others dedicated ones. |
| R-hop or special barrels | Only the parts made for that system. |
When in doubt, check the manual or the bucking maker's page, which lists compatible guns.
When to replace it
- The rubber is hardened, cracked or deformed.
- BBs curve left or right even with a clean barrel.
- The setting won't stay steady or you need more and more hop for the same result.
- The season changes and the bucking no longer works well (see the hardness table).
Replacing it means removing the barrel from the hop-up chamber. It's not hard on many AEGs, but it must sit straight with no folds: if you've never done it, have a technician do it the first time. Afterwards redo the hop-up adjustment and chrono the gun.
FAQ
Which bucking for winter?
A softer one than in summer (for example 50°–60° instead of 70°): cold hardens the rubber and reduces spin.
Is a harder bucking better?
No: choose it by the gun's power and the temperature. Too hard on a low-power gun gives little spin; too soft on a powerful gun deforms.
Are all buckings compatible?
No. Almost all AEGs share one standard, but pistols, VSR-type spring rifles and gas rifles often use their own. Always check the maker's compatibility.
Should I change the nub too?
Not necessarily, but it matters a lot: long-contact buckings are often paired with flat or "H" nubs to spread pressure.
How often should I replace the bucking?
There's no fixed interval: replace it when it's damaged, hardened, cracked or no longer holds a steady setting.
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