When the FSA does use vehicles, it prefers to use trucks mounted with Soviet-era DShK heavy machine guns or ZU-23-2 anti-aircraft cannons. Both are used widely by the Syrian military for anti-aircraft and fire support roles. The 23 mm ZU-23-2 can fire both high-explosive rounds and armor-piercing rounds, which are capable of penetrating the armor of the Syrian military's BMP infantry fighting vehicles. Although the smaller 12.7 mm round of the DShK -- nicknamed the Dushka, or "sweetie" in Russian -- is little threat to armored vehicles, it does pose a threat to the helicopters used by the Syrian air force.
Here we see a video that purports to show a DShK firing at a helicopter in the northern town of Azaz. How effective DShK fire is against aircraft at that range is questionable, especially without the detachable anti-aircraft sight that many of the DShKs in Syria appear to be missing. Nonetheless, the FSA boasts of having brought down helicopters using DShK fire, as these men from Azaz claim. This video claims to show a helicopter downed by rebels in Syria.