The real world difference is ammo capacity and reliability. Machine guns are built for both. This was part of the intent of the Stoner 63; everything is an MG receiver so you mitigate both the reliability issues in the MG configurations and bring down the per unit price of the usually far fewer MGs. The result is everyone has ~2 lb tax for the minimal benefits of commonalty with the LMG.Interesting discussion re. MGs. However, I don't see ROF 6 being any different for an LMG compared to an Assault Rifle; they are both spitting out lead in more or less equal quantities and equally fast. Maybe the problem is just that the weight of an MG and its bipod are not being given sufficient credit. Both would tend to improve the accuracy of automatic fire. Maybe just give MGs a +1 for firing when prone?
RE: dropping the Minimi for various automatic rifles, I was under the impression the BA was still using the L86 SAW and didn't adopt the Minimi until the mid-aughts HK redesign of the SA80. As previously stated, no US element would !replace it for another decade and a half after the game's time period. (The Marines still have and maintain SAWs, but the gunner has options between it and the M27 based on what the boss wants them carrying. More to follow with the ongoing the Marine rifle squad reorganization.)