Dedalus
Mercurial and fleeting
I once woke up with a 90-pound dog on top of me who had decided to make me his bed. Okay, not exactly the same situation as this but I couldn't roll the dog off and he wasn't even beating my head into the ground when it happened. I was pinned even though I outweigh the dog.
I imagine there are several 90-pound (or under) animals that would be difficult to remove from your person if they were so inclined to attack you. That is not an apples to apples comparison.
Besides, if someone is beating your head into the ground, you probably aren't thinking clearly and your first thought isn't going to be "Gee, I need to roll over and get this guy off me." Your first thought will be stop him before he kills me.
I don't see a difference in those two thoughts. You are going to be thinking only of self-preservation. It may be rolling over. It may be to reach the other's hands /arms in an effort to stop the attack or deflect the blows. Your defensive movement will be an effort to interrupt what is happening; to shield yourself. It would definitely be bent on minimizing the damage being done.
It doesn't stand to reason that in a prone position one could be taking the time to fish out a gun holstered to their waist (and laying on it, none-the less) while enduring severe blows to the head. Your arms are going to instinctively reach up to protect your head.