Weapons can be found anyway pretty much. But by look in these places the chance is higher.
Military Check Points:
On all islands, found on cross sections (4 roads cross) tents will house anything from Ammo to a Glock. If your lucky 'a commander' will be there, they have a Red Hat and shoot at you so you know when you see them. On island one they will have the basic FNX pistol (45 ACP) On island 2/3 they will more than likely have a Glock (9 mm)
For the very lucky you may find a elite soldiers 'rare' they will have a AK-74 or M4 Carbine. (The best weapons)
PLEASE NOTE: Radios can give you the locations. Found at radio towers. Random spawn. (Battery)
On island one there is normally "1" crash site normally a tan coloured jeep (for some reasons the developers haven't got the 'art' for the truck facing side ways so it will be on road going along the Y-axis (Up)
On island 2/3: more jeeps and Humvees, found in military bases and fire stations only. Helecopter crashes anyway around the map 'LUCK'
Around crashes: military gear and weapons.
Island 2 and above! Military bases. Icon on map
Have you guessed it Guns
CAR BOOTS; SUV may sometimes have shotguns in.
Misc location for guns (be anywhere on map)
Bunkers
Watch towers
Building (rare)
Guns/Ammo *= I don't know name
FNX pistol - 45 ACP clip(15)
Colt 911 - 45 ACP clip(7)
Glock - 9mn clip(18) GOOD
Magnum - 317' clip(6) (good)
Remington (shotg) - 10 cal clip (12)
//another shotgun very bad clip 2 :(
Reapeater - 7.62 clip(7)
Glospi rifle* - 7.62 clip(7)
Sniper Very Rare 7.62x32 (VPD)*
MP5k - 9mm clip(30)
AK-74u - 5.56x32 clip(30) ^F-rate
AKM -7.62x32 clip(30) ^F- damage
AK-74 - 5.56x32 clip(30) GOD
M4 Carbine - 5.56x42 GODS GOD
L82 RARE - 5.56x42 M4 basically
If you see the super 22 or the ampotations pistol don't pick them up, pea shooters basically. Your fis is better promise.