Mouse Tweak
acceleration off
sensitivity (Left to Right on your mousepad should be approximately a 360 turn)

m_pitch 0.015
(lowers up-down sensitivity to about 2/3 of Left-Right, good for headshots)
Or
m_pitch 0.010 (If .015 does nothing for you)

try to not spam <-- Holding down the trigger
- rather, fire off short bursts - this increases your accuracy significantly.
- don't set your weapon to Burst, just use it As-If (may save you sometime being in auto)

movement penalty is significant and proportional to velocity.
- so try a quick tap in the direction opposite of your momentum to reduce your speed before you shoot

vertical movement penalty is larger then horizontal so shoot at the tops of your jumps when jump and avoid strafing over stairs and what not

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This will increase your hit percentage, if you follow these tips.
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Optional:
This works best if you're pushing 125 FPS (Frames Per Second)
rate 25000
cl_packetdup 2
cl_maxpackets 42