Just remove the pop cost.
With the high cost it is too expensive to use in early game when you have the spare pop, and with the 3 pop you won't waste it building this late game when you have the resources
With the Brit it is ok because it works in tandem with their other emplacements, but even then once you get the bofor you would rarely choose an mg nest over a bofor. With the US and Wehr with no pop cap on their mg nests or bunkers, they are a very viable late game item. With the Russians mg nest as it currently stands - useless