There's a legacy ability that allows 1d5-2 when a weapon otherwise fails to cause damage. That's a special gift to a dedicated weapon.
Under your rule, an autogun, or that stubber with Storm would become the ideal Tank-Hunter weapons. A guaranteed 10 is greatly superior to the vast majority of available weapons; especially given the autogun's availability.
In fact, if you get recoil gloves and dual-wield…. at that point you should be outdoing just about any weapon that isn't already vehicle-mounted [and a decent amount that were] in average output against big fat targets like a Leman Russ' front armor or Land Raider.
It would be more reasonable for you to lower unnatural toughness a little, or grant +2 of it to squishies and upgrade weapons by 2 damage to compensate. Not that doing so is a particularly good way of dealing with the output problem of the weakest weapons.