Lighter bullet, bigger charge: why?

[mcg-doc]

Today I made a mistake while loading 38 super. I loaded 130 gn bullet over a charge developed for 145 gn. The gun wouldn’t cycle well. That got me thinking. Why do lighter bullets require bigger charge?

[somerled]

The heavier bullets require less powder to reach the same pressure level. There isn't as much powder space in the case with the heavier and longer bullets either.

[Dave R]

Another way to say the same thing...

The heavier bullet has more inertia, so it will build higher pressure at a given powder charge than a light bullet will.

[Crosshair]

Others have kind of said it. But I will tell it from the aspect of cycling you're gun.

Lets say you're gun needs X amount of recoil energy to cycle the weapon. P is powder and B is the bullet weight.

If P * B > X then the gun will cycle just fine

If you reduce one, you must increase the other to have reliable function.

If P * B < X the gun will fire, but not produce enough energy to cycle the weapon.

From what you have said, it is clear that the load is not producing enough energy to cycle the weapon. You can either increase the energy of the round or install a smaller(weaker) recoil spring.