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That's incredibly low... 340fps with a PDI 150% and 360-370fps with a 170%??
Can I ask what chronograph you are using? And is it optical or radar? Optical chronies are much more accurate and radar's usually have difficulty picking up BBs or give results that are too low or too high.
The length differences between the PDI 150 and 170 can be attributed to different materials used. A softer steel is used for the 150, but a greater length must be attributed to get it to perform where PDI wants it to perform. That material is used because there is no ecconomical grade of steel between what's used in the 150 and the 170.
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