This is from 6.0 master release notes.
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Added a strength requirement for big guns feature. Enabled by default. Penalty depends on the weight of the weapon relative to your strength. Strength 8+ allows all big guns to be used without penalty. Strength 7 can use weight 20 or less without penalty, strength 6 can use weight 12 or less without penalty, and strength 5 only big guns under 8 without penalty.
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Im guessing the penalty would have something to do with movement and or weapon sway.