I've never had a 12.5, but I've done 10.5, 11.5, 14.5, and 16 over the years. I know 556 doesn't really recoil, but I hear what you're saying.
From experience, my 14.5" mid-length with a full-weight bcg is the nicest AR shooting experience I've had. If you build a 14.5 as a pistol you don't...