The phenomenon you are referring to is known as length contraction, a concept from special relativity. According to special relativity, as an object moves at high speeds relative to an observer, its length appears to contract along the direction of motion as observed by the stationary observer. However, it's important to note that this contraction is relative to the observer and does not physically change the object.
In the case of a bullet moving at high speeds, the length contraction would be observed by an observer who is at rest relative to the bullet. From the perspective of such an observer, the length of the bullet would appear contracted along its direction of motion. However, from the perspective of an observer moving with the bullet, there would be no observable length contraction.
The reason why the bullet does not physically shrink is because length contraction is a relative phenomenon and depends on the observer's frame of reference. The contraction effect only occurs when measuring the object's length in a reference frame that is in relative motion to the object. The bullet itself remains unchanged in its own frame of reference.
It's also important to note that length contraction is generally only significant at speeds that are a significant fraction of the speed of light, which is much higher than the typical speeds of bullets fired from firearms. Therefore, the effect of length contraction on bullets fired from conventional firearms is negligible and not noticeable in practical scenarios.