Forgive me if I am wrong, but Alan's earlier comment about the PH5C's sight mount (the bit that stays on the rifle after you have removed the sight arm) covering over part of the dovetail on the BA21 rear base means that if you have the PH5C sight mount on the rifle with a BA21 rear mount, you will not be able to fit the rings to mount a scope. In other words, you would have to remove the sight mount completely before you could fit a scope. The only way you are going to have a set-up with a quickly interchangeable aperture rear sight and a scope on the rifle if you are using the BA21 rear base is to use a PH5E/4, which clears the dovetail. Then again, you would then be working on an Enforcer clone, and would have to either get hold of an Enforcer safety (rocking horse poo), grind a normal one down to fit or put up with not having a functioning safety (this has been discussed here recently).

I, of course, could be talking out of my butt as I am not actually functionally awake right now..