question on steel shot

[charlie cleveland]

ive dabbled a bit in loading steel shot and a question has poped up in my mind i can not answer...does any of the steel shot coming down the barrel of a gun get ahead of the wadding or does it stay inside the wad till it leaves the barrel when fired....if it stays inside the wad till it exits the barrel how could it possibly i cause damage to the barrel if the barrel is of a modified or less in choke...just wandering.... charlie