@armchair we don't know for sure that they are given the same force depending on the increase in weight of the Gauss rounds for the Hgauss it might be 3 or 4(If the pictures are to scale the round looks to be about 4 times as large on the CBT wiki though i take this with a gain of salt ) times heavier but there might have only been enough room to fit a slightly larger magnetic accelerate which would explain the faster drop off in dmg. Either way there is no way to tell as this is a fictional world and right now we can barely get a ship based railgun system to fire more than a dozen times without the rails melting due to the amount of damage sustained by the plasma produced by it.
FYI standard gauss rounds are 125kg.
Not buying it. The 16 inch gun, only having a diameter twice as large as the, was capable of withstanding the force required to throw a 1200kg shell further than the 8 inch gun's 152kg shell. Throughout history, we've managed to successfully scale up our weapons to keep pace with improving armor. Smaller weapons may have other advantages, but its the big guns that reign supreme in firepower and range. Unless the engineers in battletech are completely incompetent, which they very well may be since they believe that mechs are a viable weapons platform (which is just laughable from a realism perspective), I see no reason why they could not continue this historical trend.
At the end of the day, this is a fictional universe. As I stated earlier, the shorter ranges of the hvy weapons make absolutely no sense from a realism perspective. They only make sense from a gameplay balance perspective. In all seriousness, in a fight between a modern military force and the most elite clan force, I'd expect the modern military force to promptly win.