The reason for formula’s for reloads is to prevent overpressure.
Even minor changes in weight of shot or bullet with fixed powder load can be fatal.
This is an outgrowth of the days of Proof Testing guns. Back then they used black powder, harder to over load charge.
With modern powder, precision scales are a must.
Just adding shot and powder both is very dangerous. You discovered that.
At 11-12 yrs old metal fatigue is pretty far outside your scope.
Now we would be a Youtube celebrity.