What B-tech class's as a Machinegun is not what we would class as a Machinegun, In B-tech their are 20mm and 30mm weapons classed as a Machinegun, we would call them (auto)cannons (see the Fluff on the Scorpion light tank).
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.
B-tech armor is thin, at best on a mech it's only about an inch thick, yet it's perfectly capable of stopping multiple impacts that would be in the range of a battleship gun, in terms of Raw Kinetic energy. Though on the other hand it is capable of braking if dropped...
Sigh.
Okay, this is what bothers me when people mention battletech armor.
Ablative armor is pretty useless against armor-piercing weapons. The examples you provided are not effective at stopping the vehicle from being destroyed, only reducing the likelihood that an off-angle shot will penetrate. Sandbags were used as impromptu armor during WWII, mainly by US tanks, because the armor on US tanks was
terrible. A Sherman could not withstand a long-barreled 75mm cannon round due to thin armor. Placing sandbags on top of the armor where possible would not stop that same round from penetrating; it will only increase the chance that an off-angle shot will fail to penetrate by a
very tiny percent. There's a reason we don't use sandbags to combat anti-tank rounds these days.
Merkavas and similar vehicles do not use chains or cages to stop armor-piercing rounds from penetrating. A dense APFSDS round will not care if it impacts a chain before plunging into the tank itself. It's of no consequence to something moving that fast.
The chains are there to defeat RPGs and similar weapons, so that the explosively shaped projectile warhead most commonly used by RPGs will dissipate somewhat before it impacts the armor itself. A HEAT round will lose its ability to effectively defeat armor less than 2 meters after impact. A Sabot round will not.
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.
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.
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.
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).
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.
But once we start talking about thicker armor, we have to factor in weight. More weight means a slower mech, and also means that said mech is more likely to suffer catastrophic failure if its delicate system of bipedal motion is disturbed. Put a hole through a big ol' leg, and it's useless. Tanks don't have to worry about this. They can be pure firepower and armor.
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.
I'm not arguing that an Abrams could survive an encounter with an Atlas if the Atlas fired first, but I am arguing that CBT armor is only effective because weapon developers aren't smart enough to take advantage of their inherent weaknesses.
Oh, and Chobham armor doesn't break if you drop it on the ground
