Would you fire an AR-15 enough to produce flames?
These guys certainly did, and caught the forward handguard on fire.
Watch this epic AR-15 torture test that is nearly impossible to duplicate.
Sometimes you just have to push a weapon to its breaking point to see how well it will work. Who wouldn't want to set an AR-15 on fire, honestly?
It took lots of ammunition to test how well this rifle will hold up to heavy sustained shooting. All of this "pew pew" causes the handguards to burst into flames. A quick toss into a mud puddle puts out that fire.