This trick requires the dual handguns, all weapon slots full, and at least 
one bullet for the dual handguns.
Have the dual handguns equipped, and find a weapon you do not have in your 
inventory. This weapon
must be on the floor, not leaning or on top of something. Hold Square to 
switch out the dual handguns
for the new weapon, then switch back for one handgun and grab the other. You 
will notice that you have
more handgun ammo. For example, if you had one bullet for both and you used 
this trick, you should now
have two bullets. If you repeat the trick, it will increase to 4 then 8, 16, 
32, 64, 128, then the maximum of
200.