I disagree about the G18, I think it's no worse than the M27. The M27 seems to have an abnormally high rate of one-hitting people with full health, for reasons unclear to me. There were six opponents in a tunnel once and I wiped them all out with the m27 within about two seconds.
Although I will admit that every now and again the G18 gets lucky and gets good accuracy even at long range, it usually doesnt work quite so well. Maybe increase the recoil a bit, that could help, cause it's really the ONLY gun that's effective at short range.
Either that or reduce the bullet lifetime so it can't be used at long range. I've heard that due to the stuffty scripting of the weapon pack the Rapid Fire mod works with the M9 and the G17, allowing an MG that looks like a pistol and never has to reload. This is rendered considerably less useful by the fact that it's impossible to Rapid Fire and Zoom In at the same time, preventing stuffty MG-sniping in many cases.