Looking for opinions on which rifle to purchase as the basis for a "SHTF" platform and which will have a Crye Precision Six12 18" barreled modular shotgun attached under the barrel. So far the two options that I'm considering are;

1. 516 Carbon TS

2. MCX modular carbine in 5.56

Who has any thoughts/opinions?

EDIT: I should add the following - I'm not looking for any SBRs/other NFA considerations. I don't need the info ASAP, as I'm still in the planning stages of this particular project/build, but it never hurts to gather as much info as possible before I pay for anything.

Like a Masterkey?

Your gun, your money, your choice, of course, but unless you're on the sooper-tacticool dynamic duo entry team, they don't serve much use.