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Bore Up kits are generally used for longer than normal barrel lengths, or for high velocity setups.
Pneumatic efficiencies are much more glaringly apparent above 400fps with exponentially higher air losses when shooting with an M120 or higher. Thus, bore up kits are sometimes suggested as a way to reclaim lost air volume passing through the inner barrel at these higher velocities.
As a side note, bore-up kits should only be used with M120+ springs - I'd recommend you begin considering it once you step in to M130 territory. Springs weaker than that may not have the strength to expand the o-ring enough during the compression phase.
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