copy/cut the weaponsinfo.xml document into 
C:\Program Files\Rockstar Games\Grand Theft Auto IV\common\data 
and when it sez to replace the previose document click copy and replace(for vista users)
but i don't know what to click for xp users, but just replace it.close that window and start playing/enjoying.

IN THE MOD

Knife: the same
Pistol: slightly weaker
Pump Shotgun: shorter range, more powerful 
Uzi: fires nothing, handy when facing enimys with them
AK47: fires rockets 
Sniper Rifle: the same
RPG: the same
Molotov Coctails: the same
Baseball Bat: the same
Deagle: fires rockets
Combat Shotgun: longer range
Smg: one hit kills, two hits to make a car catch fire
M4: single shot, handy if those bloody cops are after you
Combat Sniper Rifle: the same
Rpg: the same
Grenade: the same
Object: the same

ENJOY!