"its about time NCOs got their UBER SPECIAL GUN AAAAAAAA"
Sounds cool, but ultimately unnecessary. MK4's been in the dumpster for nearly a year and nobody left the server over it.
There are also additional problems with designing an anti-arachnid rifle:
1) It would have to compete with Mark Two, the resident arachnid killer, for it's role to be justified. When roles overlap, it's very easy for one gun to overshadow the other. Perhaps this could be solved with careful tinkering of numbers to give both weapons a niche, but there are far more ways to fuck it up than there are ways to do it right.
2) Even if it did work well, do you really want infantry power vs arachnids to get a flat increase for everyone except the pfcs? Imagine being an admin and complaining about those two stupid fucking marauders melting your warriors too easily. Now imagine that complaint applying to the entire server. I can see the natural consequence of this being higher HP arachnids in the future, which would take their TTK back to where it is now, sans the one recruit who hasn't had their cert yet, and has to stick around for their promotion despite having an absolutely miserable time trying to kill anything at all.
I agree with the problem you outlined (Weapon specs hijacking progression for enlisted who arent NCOs), but your way of going about the solution (make better MK2 but Pfcs still cant use it) is very roundabout and doesnt actually adress the Pfc issue. It just gives NCOs something new to shoot.