will Real 870 fore-ends fit on an airsoft CA870?
 

I'm wondering if the parts are interchangeable, especially the fore-end.

Out of the box? No, but its close. you will need to dremell the grip to fit because the location for where the cocking bars meet in a different spot.
Even then it's gonna be a little loose in spots

Interesting. Would be curious to know if it works the other way at all then, too.