According the Nelson, the guy who heads the factories, had mentioned that there is not enough material within the Mk3 body to make room for the square head magazine.
The Mk3 is essetinally the Mk2 body with the gaps for the cocking handle and power tubes covered.
The HEAT core being standard and is mag-fed ready like mk2.
I don't know if the Mk3 has any weight reduction done to it. The Mk2 body was much lighter than the Mk1 and the Paradigm 1.
The idea of the new CQB mk3 is basically the offerings of the new internals in a ready to go, affordable package.
And then down the road they'll release the Elite editions like they did with the previous series.
More official updates to come in the following weeks about pricing and what not.