Could this shoot slugs?
If size appropriately, sure. Though I doubt it will produce the velocity needed to stabilize them
Understand that is is shrouded. Is the shroud end threaded to accept an adapter and to add a moderator?
Yes it is threaded. DonnyFL makes a nice adapter that would allow for a more standard threaded "extension" to be added.
I have an Ataman AP16 .22 caliber pistol. Are the magazines used in the AP16 the same as the BP17?
Looks like the end cap of the rifle could be removed? If so, do you know how it is threaded?
Yes, it is threaded. I do not have the size, but DonnyFL makes an adapter you should check out.
Can we add silencer?
The barrel is in fact shrouded, but an adapter can be fabricated and further silencing capabilities can be added via all the usual means: DonnyFL, Neil Clague, etc.
The barrel is already shrouded.