I also have a G17c and the purpose of the compensators is to reduce recoil which it does regardless of you not noticing... To tell this just go shoot 100+ rounds with a compensated and compare by shooting a non compensated... Your arm won't be as tired and have shooters fatigue when using the compensated... It discharges the gas created through both the vents and the end of the barrel instead of everything just through the end of the barrel - results in less muzzle flip for even more rapid firing (preferred by those in competitions for that reason) | 
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