Originally Posted by FABIO GETS GOOSED!!!
Here's another wager proposal: OAL agrees a legally tenable argument can be made that a Bullet Button equipped, semiautomatic centerfire rifle has the capacity to accept a detachable magazine, notwithstanding APA-compliant, binding 5469(a) and the lawfully configured SKS fixed-magazine rifle.
I think I know where you are going, except I don't think a legitimate argument can be made by your reasoning.
In order for a judgement to be made whether a specific rifle is an "assault weapon" it must be considered as a complete, ready to fire unit. That includes a magazine inserted in the magazine well. When using a properly installed Prince50 or BB, with the magazine inserted, it cannot be removed without the use of tools and no longer has the "capacity to accept" any magazine.