Sorry its taken me so long to reply.
How exactly are you using this script? an activator or something else?
Edit: This script will work, just attach it to your activator.
scn AmmoSortingScript
Short Playercount
Begin OnActivate
Set PlayerCount to (Player.GetItemCount Ammo556mm)
Player.RemoveItem Ammo556mm PlayerCount 1
houseammobox4.AddItem Ammo556mm PlayerCount 1
End
I hope this helps.