Actually, Mech machine guns are analogous to .50cal machine guns. AC2 and AC5s would be classified as cannon weapons, hence autocannon. A machine gun stops being a machine gun after it reaches a certain size. Perhaps this is changed in THE FUTURE™, but all evidence points towards standard MGs as being simple .50 caliber machine guns.
Did you not read my post, B-tech has a Tank that uses a 20mm MACHINEGUN, and mentions that their are others with 30mm Machineguns. And I can point to more...
And then Evan stumbled backward as the Ranger’s machine guns opened up, slamming twenty-millimeter
caseless into the ConstructionMech’s bulky frame.
There, a cadet-crewed Schmitt probed forward, found a Triarii infantry position exposed
and hammered twenty-mil rounds into their position.
The Di Schmitt has two ballistic weapons a pair of Mydron Tornado RAC-5s and a pair of Scattergun Machineguns. The RAC-5s are described earlier in the book as being 50mm, leaving these 20mm weapons as the Machineguns.
Some I can see. A whole lot I can’t. Gun trucks with machine
guns and grenade launchers... ...His gun truck bounced over brush and rocks as it shot forward, three more swinging out in rough
echelon as they zigged and zagged behind him. Gunners hung on to their 20mm Gatling guns attached to
the roll bars on the enemy side of the trucks.
Armament:
3 Holly SRM 6
4 Holly SRM 4
2 22mm Gatling Guns
The TRO entry lists the 22mm Gatling Guns as Machineguns.
The 20mm Gatling gun is a time-proven weapon, giving the Scorpion good defensive firepower with plenty of punch. Though some users of the tank have tried 30 mm machine guns, the Gatling gun's high rate of fire makes larger shells unnecessary.
Still think B-tech Machineguns are only 12.7mm Weapons?
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If you think about it, a Mech is the ideal target for a tank. A mech is massive, slow (for anything beyond a light mech), and very easy to spot at long distances. All combat is first-shot, first-kill. There's no reason to believe this will change in the future. Modern armor is not simply steel, like HEAP rounds would be useful against. Modern armor relies on layers of sloped, alternating materials to form a super-dense wall of protection. All impacts will either be diverted, if an off-angle hit, or absorbed and defeated by the armor itself.
This may be true in real life...
Take my Abrams, for example. The newer version of Abrams armor is multi-tiered: Reactive armor that explodes on impact, thus redirecting the energy away from the armor. Beneath that, sloped Chobham armor, which consists of alternating layers of steel, ceramic, depleted uranium, and flexible meshes that fully absorb or redistribute impacts.
B-tech armor is a composite as well, Steel and CBN (with diamond reinforcement, on one or both layers, Titanium as structural, with a polymer sealant) in fact. Also the Internals is a Titanium honeycomb. Oh did I mention that the armor and internals are laced with sensors? And a single ton of armor can be replaced in as little as 2 to 4 hours.
And no Abrams have reactive armor unless it has under went the TUSK upgrade, and then they only cover the treads, not the turret, front or rear hull. And that theirs only like 500 Tusk kits available means only a small number have them.
Mech armor is not sloped. In fact, most potential impact zones on a mech are vertical. Vertical impact zones mean anti-armor rounds will have the maximum amount of area to impact, virtually guaranteeing penetration. If, as you say, the heaviest of mech armor consists of a single inch of ablative material (and I would assume a steel-like compound beneath that as skeletal support), there is nothing an Atlas could do to stop it from being punched clean through by a depleted uranium penetrator moving at a kilometer a second. Projectiles above AC range tend to be large and slow. Even Gauss slugs in CBT move no faster than a modern Sabot round, and they don't have nearly small enough of an impact zone to effectively penetrate armor.
No B-tech Rounds are much much faster than real life rounds, Gauss Slugs are hypersonic, as fast if not faster than real life rounds which top out at around 1,800m/s (just over the 1,710m/s for hypersonic). Not to mention the ranges in space are canonical and their is no difference between a Gauss Rifle in space and on on the ground. A Gauss Slug may not be ideal shaped but it's mass will spell doom for any real life tank, 225+ Mega joule impacts are not something real life tanks are built to handle (FYI thats equivalent to a 14 inch battleships shell), an Atlas can take three of these hits to it's center torso and not have any internal damage. And if you do not believe that B-tech Gauss Rifles are Hypersonic, Well I'm gonna have to post all 39 quotes that I have that say yes they are... (Heck I even found some infantry weapons that are hypersonic, including a 12.7mm rifle...)
HEAP rounds, like those used in BattleTech, are old technology, useful on large amounts of standard steel armor such as battleship plating, but not as effective as modern kinetic rounds like the APFSDS (aka Sabot).
Perhaps Sabot rounds are not as effective on B-tech armor as you would like? Though B-tech rounds are DU tipped, travel at hypervelocity (thats faster than 2km/s) speeds and often do seem to have a HE component.
BattleTech armor relies on ablative armor, which is not surprising given the use of directed energy weapons. Armor that flakes away under intense heat is ideal for dealing with heat-based energy weapons. Said armor is also supposed to "shatter" upon contact with kinetic impacts, which is all well and good, except that for purely ablative armor to be able to absorb multiple impacts, you must either: a) have a lot of it, and thus thick layers of ablative armor, which completely negates the idea of a single inch of ablative armor stopping multiple battleship-sized impacts, or b) have a way to very quickly replace the ablative armor, nearly instantaneously. Since there is no evidence to support "b", "a" is more realistic.
B-tech armor on mechs is at best an inch thick, and can stop a pair of Heavy Gauss Rifle slugs (on the CT) traveling at hypersonic speeds, thats at lest 380 Megajoules of Ke each and likely far higher. And some B-tech Tanks can take upwards of four of them to the front hull and live to tell the tale.
It doesn't matter how advanced your armor is, or how many snazzy futuristic words for steel or ablative armor you throw around. A dense, dedicated anti-armor projectile moving at a kilometer a second WILL defeat it.
What about the fact that B-tech rounds have a velocity more than twice that of your 1km/s rounds?
Not to mention B-tech has a weapons thats smiler to real life weapons, and is fluffed to have been used until the AC-5 was introduced in 2250, their not very good on Mech armor and are not very effective in space...
