The only exploding bullets I can think of are the 50 cal API, or the pure incendiary - and they don't really "explode".
But the thought of an exploding bullet makes me think of the need for a fuse and the problem of keeping a bullet nose forward at longer range. It does not happen that way.
Exploding bullets require a contact fuse of some kind. At long range small arms bullets display all the characteristics of a gyro, creating problems for bullets that should arrive point first.
First, think about a gyro. It spins and has stability in space. A bullet has terrific rpm and thus has gyroscopic stability. There are degrees of stability. Just barely being stable RPM is called “S”. A well made match bullet should have about 1.3xS.
The amount of rpm required to stabilize a bullet is a function of bullet length and air density. The faster a bullet travels, the more it compresses the air, so more RPM is required. But as it picks up RPM from the barrel, launching it faster automatically gives it more RPM.
As a bullet slows down as it goes down range, it needs less RPM to be stable, because the air is less dense, so as it moves downrange, it gains stability. A bullet with 1.3S at the muzzle might have 2 x S at 1000 yards. So, if you have enough twist to get accuracy at 100 yards, you have more than enough stability for long range.
This is where it gets interesting. If you have ever played with a toy gyro, this will be easy to follow. If you have not, I will explain how a gyro works, first.
If you hold a spinning gyro and move your hand around, it will stay stable in space. That is, it will move on its axis so as to always be pointed the same way. If you force the gyro to turn, it reacts by turning on the other axis. i.e. if you turn it right, it will twist up.
When you fire a bullet at a long range target, it is aimed slightly upwards. It is a spinning gyro. Will it tip over, like an arrow, and strike point on – or will it come down, still pointing up, the way it was launched? Depends.
If it was launched with more than 1.5 S, it will stay pointed up, and come down with the side exposed to airflow. It will shed velocity more quickly than a bullet that tips over and comes down point on. This is why long range target shooters use barrels with just barely enough twist.
BUT, a bullet that tips over, like an arrow, is still a gyro. When it rotates over, it also turns to the side. So it, too, shows some side to the airflow.
