Technically you are correct, What B-tech class's as a Machine gun should be Autocannons (as many "MGs" are 20mm cannons), but your talking about something that's taking place hundreds of years from now... So perhaps they have a different definition for a Machinegun...
Well... that's the point - hundreds of years from now, the amount & size of projectiles required to destroy the armor is far far greater than currently. Novadays a bomb in the size of single Tbolt is more than enough to destroy even the heaviest tank, while it's not the case neither in MWLL nor BT. Novadays single round of 30mm gun can pierce through the APC, while it can't in MWLL nor BT. Novadays single 13mm round bounces off heavy tank armor as if it'd be solid rock - yet it harms the armor in MWLL - LOL. Something it's right here.
BTechs future armour is what's causing the confusion here, a very thin layer of this stuff is able to absorb and translate megajoules of energy and kinetic impact across the entire area of the contoured armour plate, you might hit the knee but the lasers beam will be ablated against the entire leg area for example, but this armour is actually VERY fragile in other ways (thus melee combats popularity in the Inner Sphere where we have heavily armoured 'mechs but don't have the sheer firepower of Clan machines). Even a Light 'mech would be the doom of a modern day company of tanks (their massed firepower at optimal range would not even add up to one AC10 burst worth of damage to BTech armour) The 2 downsides of this amazing stuff are:
1: It degrades with ALL damage, so things like Machineguns and Infantry rifle fire (20 Riflemen concentrating fire equals 1 vehicle class mgun IIRC) will slowly penetrate even the heaviest of armour.
2: Crushing damage, such as falls and collisions with terrain or hatchets are perhaps the 'mechwarriors biggest threat, as low energy impacts like these can cause entire slabs of armour to break off and fall in shards from all machines.
A previous poster mentioned how this stuff is similar to modern day armour plate that can stop bullets but will break if you drop it is a pretty good equivalent....just imagine that 1000 years from now this plate will also stop energy beams and battleship sized shell impacts whilst loosing a few microns-centimetres of thickness with every hit until it's gone.
Off Topic: I loved interstate '76 and it's heated seats but I honestly don't want anything like that in MWLL, I want it as close to a SIM as possible, aesthetic upgrades sound great, but anything that can give even a 5% advantage to damage or range or speed or in CoD MW style upgrades I do not want, pilot skill and tactics should be our only advantages I believe.
Well their is always the Capital Scale ranges which are cannon but often gets ignored. As such a Machinegun has a velocity of at lest 3km/s (108km in 60 seconds or less).
Also the old fluff on the Mackies first combat trial (It's rather old dating to the mid 80s IIRC) mentions an 105mm AP round with a armor penetration of roughly 35cm (decent for the 105mm during the early 80s) being stopped cold by prototype armor with a thickness of no more than half an inch (the plate was as thick as a mans finger).
The Mackie was the first battlemech by the way.
Many deride it for the low AP performance but most fail to notice that what stopped it was at best a half inch of armor that is obsolescent in current B-tech times.
But I do not know how cannon this quote is any more, it may have been retconed... (I do know their is newer fluff about this but I have not seen it at this time).
