Back stabs are only made for 14 gauge wire. A long time ago, you might be able to cram a 12 in there, but not any more.
Anyway, back stabs are evil.
I agree with
rlitman
Let’s expand on the “evil”...
Those back stab style receptacles are the sources of many problems not the least of which is loosening up over time to become a source of overheating and even fire. You definately don’t want fire inside your walls.

Personally, I use the type of outlets that have screws on the side which tighten clamps that grip a straight piece of wire inserted from the back of the device. To me, that saves time and the aggravation of forming a tight loop and getting it properly tightened down under a screw.
And the devices made with that feature seem to be better made internally and not prone to wearing out.
Of course there are millions of those other under a dollar each devices that are in service and have not failed. “ Your Mileage May Vary”