Does anyone know if the Magpul MOE mid length forearm will use (when it is released) the front end cap on existing barrel assemblies? I know it does use the delta ring.
Well, the 7" carbine version uses the standard endcap.
I have to assume that the midlength and rifle MOE handguards will fit triangular caps. Otherwise, Magpul will be creating a handguard system that will fit very, very few firearms. 
Makes sense.
I will venture a guess that it will fit both the round and triangular caps, in a fashion similar to the Cav Arms mid lengths. 'Just a guess, though.
Second question; Can anyone tell if it will allow the use of the side mount sling attachment? The ones that mount in-between the post’s of the standard front sight base. The current shorter model does not look like it will. I see they will make the new ones taller to accept piston uppers.