This is what I came up with:
A BBB that loads itself when cocked. It simply opens a breech with a magazine over it when pushed forward, and closes off the breech when the gun is cocked.

The breech is open.

And then it is closed.
This may have been done before, and I know someone wanted to try something similar with a NF, but I haven't seen this actually done on any other gun.
Questions? Comments? Snide remarks that accomplish nothing?