Ok, here goes.  I did a search and only got 2 hits on this topic.  So...does a detachible mag count as one evil feature or not ?  In other words, can I put on a collapsible stock on a postban AR that has no flash hider ?

A detachable magazine is not 'a' feature - it's one of the defining features.

A semi-automatic rifle made/assembled after 9/13/94, with the capability to accept a detachable magazine may have at most ONE of the following:

pistol grip,
folding/collapsing stock
bayonet lug
grenade launcher
flash suppressor/threads for a flash suppressor

If a post-ban rifle has TWO listed features, it's an illegal 'semi-auto assault weapon'

The standard post-ban AR is a semi-automatic rifle with the ability to accept a detachable magazine, so its usual ONE feature is the pistol grip.  If you remove the pistol grip, you can have a collapsing stock.

If you make the magazine permanently non-detachable, you can put anything on it you like - including the flash suppressor and collapsing stock and bayonet lug.  Some state laws (like California) are more restrictive, and won't allow the pistol grip or flash suppressor regardless of magazine status.

Circuits, you are wrong about the Caliban. If you have a non-detachable magazine which holds 10 rounds or less you can have the pistol grip and flashhider. It's no longer considered an AW by the Caliban.

BKinzey:  Ah, yes - I oversimplified.  If you have a nondetachable magazine of more than 10 rounds, it's still an "assault weapon" in California (even though it would not be one under federal law), but a nondetachable magazine of 10 rounds or less would exempt it under both Cali and federal law.