Do all bullets mushroom when they penetrate a target?

Answer 1

No. Some explode into fragments, while others remain intact with little deformation. It mainly depends on the target, the velocity of the bullet, and the composition and shape of the bullet.

Plastic deformation in soft metals is well documented, and bullet designers try to one or more goals in a bullets design. Some bullets are made to penetrate very deeply with little deformation (full metal jacket bullets as used in battle). Others are designed to deform to 2 to 3 times their original diameter while staying intact to transfer as much energy as possible to the target without fully penetrating it, hollow point handgun ammunition, and large game heavy bullets. Other bullets are designed to fragment into small particles upon impact usually for varmint hunting, or for safety issues when shooting in populated areas (glassier safety slugs).

A low velocity bullet that has a full metal jacket penetrating very soft target may show little deformation. This might be true in the case of ballistic Gelatin being used for a target during forensic analysis.

On the other hand, a full metal jacket hollow point .22 caliber, going 3000 feet per second or faster will probably explode into tiny fragments upon hitting even a soft target such as ground squirrel.