White Hot 50 Cal machine gun

My platoon Sargent thought he saw an enemy soldier out some distance from our fire base so the decision was made to work the area over with a 50Cal. We were doubting his report but decided to take action anyway. 

At first the gun worked fine and the tracer rounds traveled in nice smooth arcs. A number of rounds later the gun barrel turned red hot and later yet it was white hot. Then strange things happen to the tracers. They began to travel in circles and they even seemed to turn corners. At that point, we stopped and let things cool down.  

The next day a discussion began which I carried over into my Science classes for the next 40 years after my Army service: "Did those tracer bullets travel erratic paths because the heat made the bore of the gun barrel get smaller and pinch the bullets, or get larger and not put proper rotation on them?" (fyi  thermal expansion caused the barrel to expand so the bore got larger)