The AR (M4) is really THE SHTF carbine if you take into consideration that it spans the ranges (0-600 yards) with a dual scope setup. Even with just a dot, MOM hits are easy out to 300 yards.
I started out with an AK and T1 as my SHTF gun because I fell prey to the "reliability" argument on the internet, but sold it and have not found a better SHTF setup then my current one which is a quality 16" barreled (Criterian .223 wylde) AR with a TA01 Acog and T1 offset with a Larue mount.
The T1 rules at 100yards and in and for moving targets, while the 4x with BDC makes first COM shot hits out to 600 yards "plug and play" and 1MOA hits to 300yards no problem. The ability to switch near to far with this setup is the icing on the cake.
One gun handles all my needs, makes it easier to decide which one to bring if I have to bugout. There is a lot of coin tied up in that setup as are most ARs (even if you build from parts like I did) so unless you have the $$$, I always suggest an Remy 870 for a SHTF gun. Slug hits to 100 yards with a bead are easy and with a california competion loader you can feed them pretty fast with practice.
Which brings up another point. The Grandson was talking about wanting an AR15 the other day (which he can't afford, or won't prioritize I should say) but without training it really doesn't matter what he has. A good shooter with a Win 94 is much more leathal than one that doesn't practice or know his gun even if it's a Noveski M4.
Run what ya brung, just make sure it's not your first race.