Islamic State has many US weapons including some that were sent to Syrian moderate rebels

Many weapons used by the Islamic State were seized as they captured bases in Iraq from Iraqi forces. However, the group also has US weapons that appear to have been given originally to moderate Syrian rebels linked to the Free Syrian Army.
  Obama has always been hesitant to send many weapons to the rebels of the Free Syrian Army fearing that they might end up in the hands of militants. The western-supported rebels have constantly complained they are not as well-armed as more militant groups. However, some US weapons provided to moderate rebels have ended up in the hands of the Islamic State in any event.
Some Islamic State US weapons may have been obtained when they took over territory controlled by the FSA but some also could have been US weapons provided by Gulf States such as Saudi Arabia to the Islamic Front. The Islamic State has been in constant conflict with them as well.
 Researchers with a group called Conflictarm embedded with Kurdish forces both in Syria and Iraq during a ten day period in July were allowed to catalog Islamic State weapons captured after clashes. Not surprisingly the weapons were manufactured in a number of countries: Along with the anti-tank weapons, manufactured in the former Yugoslavia, researchers documented a handful of U.S. M16A4 rifles, two Chinese Type 80 machine guns, a Croatian sniper rifle, a 9mm Glock pistol and various Soviet-era small arms. The Islamic State has also captured some territory and won out in some battles with Assad forces where it captured Soviet small arms. Some of these items could have been bought on the black market. It is also quite possible that some of the middle men ferrying weapons from the west and Gulf States to rebel groups are pocketing extra money by selling some of them to radical groups such as ISIS.
 Researchers found that anti-tank weapons used by the Islamic State had previously been sent to Syrian rebels. A number of weapons had serial numbers welded over in an attempt to foil any attempts to trace them back to their source. This may indicate the weapons came through a third party trying to mask the source of the weapons. Syria rebel factions who receive US arms have denied that they have provided any arms to ISIS. They insist that the overwhelming majority of the US aid is used by the Free Syrian Army to fight Assad forces. This may very well be true, but the FSA is an umbrella group and probably contains some not above lining their own pockets by selling some of the US arms to the highest bidder.
 Obama is asking the US Congress to greatly increase funding to supply the rebels with more arms and other support. He intends to use the rebels as proxy forces to attack the Islamic State as Kurdish peshmerga and Iraq security forces are being used in Iraq. In both countries any Islamic State successes will gain them more US weapons:
According to one White House aide, the president is seeking more aid for the rebels so they could be the ground troops in place to support potential U.S. airstrikes against ISIS. The Obama administration already is pursuing a similar strategy in Iraq, where U.S. airstrikes are backed by Iraqi security forces on the ground -- as opposed to U.S. ground troops
 In another issue involving moderate Syrian rebels, a spokesperson for the family of beheaded journalist Steven Sotloff claims that he was actually sold by them to the Islamic State or ISIS: Sotloff family spokesman Barak Barfi, a foreign policy research fellow at the New American Foundation, made the startling claim in an interview with CNN’s Anderson Cooper.“We believe that these so-called moderate rebels that people want our administration to support, one of them sold him probably for something between $25,000 and $50,000 to ISIS and that was the reason he was captured.".