Why did my bullets start tumbling?

Question

I bought a new Beretta U22 semi-automatic pistol and took it to the firing range. Between a friend and I we put about 400 bullets through it in about an hour and a half. Then it started to lose accuracy. Before we were done the bullets were tumbling all over the place, making it impossible to get any accuracy even at ten yards. I was using subsonic ammo. Whoever gives the correct answer first wins.

Answer 1

The gun became fouled with powder residue, jacket material and lead.  Filling the rifling grooves without rifling the bullet does not spin and is not stabilized.

Answer 2

Because you were not cool enough to keep your bullets from tumbling like they do in the movies you should have done some kind of slow motion spin move and walked up the walls while firing this would have increased your accuracy greatly.

Answer 3

You fired way too many rounds and especially using subsonic bullets... sorry mate but your gun is destroyed

Answer 4

400 rounds in an hour and a half...?  Your gun must have gotten pretty hot.  I usually just get off 100 to 150 in an hour at the range.  The heat must have caused the barrel to expand.  You said that it fired fine again later, so it must have cooled back down afterwards.