Laying the bbl on your 6' level flat surface has shown the bow/bend in the bbl by checking the opposing flats.
Just bend the bbl back into shape.
Placing the very ends on the bbl on blocks and with the bow/bend of the bbl 'up',,you can use simple hand pressure and body weight to bounce the bbl in the center and bend it.
Like any piece of metal, especially a long , evened dimensional one like this ML bbl, you can feel the piece give in to the pressure when it bends a small amt. Keep checking for straightness with the flat surface of the 6' level.
(CF rifle bbl are much more difficult to bend by this method as they are dimensionally all over the place from breech to muzzle. Plus they are usually a tougher alloy steel.
They will not generally bend easily and exactly where you want them to. A press set up is the better choice for these for better control)