how gun remembers that the round chambered is a different type I leave up to you smart folk.

There's not really any need for loose ammo other than for those guns that don't use magazines. Working within the existing system we just need empty magazines that can be refilled, ammo crates/boxes already act as non loaded ammo it doesn't need to get any more complicated than that.

Likewise there's no need for a chamber attachment, the logical thing to do is increase the ammo capacity of any semi-automatic and automatic weapon to +1 over the maximum magazine size.

Then when you load a full magazine while the gun still has 1 round left, an empty magazine is created leaving the gun with max ammo count +1. If the previous magazine contained a different ammo type then that data is transferred to the created empty magazine

Now the question is how to create an empty magazine, rather than create a whole new set of items in the xml list, what is needed is an invisible bullet. Have the game add one to the ammo count of every magazine.

This should be invisible to the player and weapon so a 31 round magazine still displays as 30 and only fires 30 (unless there was 1 round already chambered). So weapons can have plus one over their max capacity but can only load their max capacity.

I know this wouldn't allow for everything the OP was asking for but it wouldn't require any new items to be created.