Here's the listing of all of the abilities of each rank.
Note - all ranks automatically have the benefits of the ranks below them.

Psykotik

2nd Lieutenant
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No benefits.
60% speed.

1st Lieutenant - 250 EXPERIENCE
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Takes 95% of normal damage.
Accuracy increased 50% while crouching. 
Deals 110% damage.
90% of normal spread on bullet weapons.
80% speed.

Captain - 500 EXPERIENCE
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5% chance of not being spotted by monsters.
Player trail coordinates added with less accuracy (10 units).
Deals 120% damage.
80% of normal spread on bullet weapons.
No kick on machinegun.
88% speed.

Major - 1000 EXPERIENCE
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Longer underwater breathing time.
Adrenaline adds 2 health to max instead of 1.
Deals 130% damage.
70% of normal spread on bullet weapons.
Full speed.

Lieutenant Colonel - 3000 EXPERIENCE
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15% chance of not being spotted by monsters.
Takes 90% of normal damage.
Deals 140% damage.
60% of normal spread on bullet weapons.