Well, interia is momentum, its just an american way of describing the same phenomenom. the 'speed of light' is the fastest any ojbect may ever go in a vacuum, it is only 'coincidence' that light travels at the same speed. This speed is not limited by mass, but by it being the very fastest possible. Also, the field you want to be looking at is special relativity (which covers light, relativistic speeds, and the effects: lorentz contraction, time dilation), not general relativity, which mainly covers spacetime and gravity. I've never seen the equation you have stated, and I'm currently doing a physics degree.
The equation is from wikipedia and the basic one for relativistic momentum.
And photons, or light, DO have mass, and they do have momentum. They have zero rest mass, but they have relativistic mass in terms of e = mc^2.
If inertialess drive meant truly inertialess, in terms of general relativity inertia, I believe that they might be able to go faster than light.
Actually, if inertialess dirves could give you 0 relativistic mass, that would imply from relativity equations that if you went at the speed of light your mass would be 0/0, but the equations would allow you to go faster than light (speed no longer plays a role in your mass, therefore there is no bound on your speed).
I thought we didn't, and that Imperial Science had no way of explaining guass flayers, not they didn't know where the trigger was.
Necron codex, imperial techpriest says that gauss flayers work by using... an EM field or something to literally strip away a target molecule by molecule, player by layer. This is why when someone gets hit by a gauss flayer, first their armor seems to peel off, then their skin, flesh, and finally their bones disintegrate (or the necron unit has stopped firing and moved on).
It is also why necrons can penetrate any armor given enough time, or wound any creature - nothing is strong enough at a molecular level to resist a gauss weapon. Stronger armor simply means each layer is stripped slower.
There is also a pict-capture of a regular necron destroyer penetration both sides of a land raider with its gauss cannon. The techpriest notes that to be able to penetrate both sides of a land raider, without deflection, would require the equivilent energy of an imperial titan or starship. Techpriest then goes on to comment on how he is going to pee himself seeing as necrons can mount a weapon of this magnitude on a light skimmer.
Of course, this takes tremendous energy and the techpriest would love to know how they do it, but has no idea of how this effect is actually accomplished, ie. how so much energy is generated or how the field is projected.