I would suggest an entirely different approach when it comes to damage, armour and penetration: raise the numbers
Hitting an unarmoured person with a solid projectile weapon in head, chest or groin is quite deadly. Almost always. Meaning a stub revolver should deal enough potential damage to kill an average Joe with TB 3 with a single shot!
A stub revolver with 1d10+10 damage? Why not?
On the other hand, modern armour should give adequate protection against primitive weapons. Like a stub revolver. For instance it should be virtually impossible for a stub revolver to penetrate Power Armour.
Power Armour AP 25? Why not?
When you are wearing power armour, it shouldn't really matter whether you have a TB of 3 or 5. Your body is made of bone and muscle, Power Armour is made of ceramite plates.
A melta gun is built to penetrate almost anything? Even Power Armour?
Melta Gun Pen 30. Why not?
I know that big number can get uncomfortable. But they can be fun, too! Don't be scared of them!