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It's the calibration result. Ballistic gelatin requires a .177 steel BB to fired into it to ensure the proper density. The acceptable range is 590 fps +/- 15 fps and 2.95"-3.75" of penetration. Calibration results that fall just outside that range can be adjusted by the McPherson method, if you subscribe to that belief. Test results that fall within that range can be taken at face value. Test results that claim calibration but do not give figures should be taken with a grain of salt.
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