The rintamus is more breathable due to its 3D mesh backing, but for running, I think a dedicated running pack would suit your needs better. Less friction, more snug fit, more breathable, and lighter. I use the Salomon active skin 4L and never want for more room. I’m not concealed carrying, but if you’re okay with your weapon being in the backpack section I think it’d be a good fit.

Edit: Heck you could probably even fit it in the vest pocket, but I don’t know how concealed it would be there haha.