The baseline is this:
BAR (Barrier Armor Rating) in the BattleTech war game is normally not tracked because it's largely presumed that all units are using military-grade armor. At the scale where Mechs and vehicles are regularly hurling heavy weapons fire at one another, the effect of BAR is normally not tracked. But, of course, not all armor is equal; personal armor, the armor used on battlesuits, and non-military/light support vehicles don't typically do well at stopping a Mech-mounted large laser or autocannon blast.
Since the role-playing game focuses more on what your characters may be doing outside of the cockpit than in it, where personal weapons--which are way less powerful than the heavy weapons mounted on Mechs and combat vehicles--are much more common, the difference in scale for such weapons is expressed by an armor penetration (AP) factor. A weapon's AP vs the target's BAR determines how well they do vs varying degrees of armor (from leather jackets at BAR 1, all the way up to battle armor at BARs of 6-10, and Mech-grade military armor at BAR 10). A hand pistol with an AP of 4, simply won't do much to the armor of a true combat vehicle or BattleMech, which typically has BAR 10 armor--no matter how much damage it could do to flesh and blood--because the BAR is so much higher.
How it works is thus: A higher BAR than a weapon's AP generally reduces the weapon's personal-scale damage by the difference, so shooting a high-powered pistol with an AP of 4 and a BD (Base Damage) of 5 at a target with BAR 6 will reduce the base damage the pistol delivers by 2 points (BAR 6 - AP 4 = 2), to a modified damage of 3 points (BD 5 - 2 = 3). The same pistol against BattleMech armor (BAR 10) would ping off with no actual damage unless the player got a REALLY high Margin of Success/lucky hit, as BAR 10 would subtract 6 points from an AP 4 weapon's base damage (BAR 10 - AP 4 = 6).
Vehicles typically used at the personal scale have "armor" that reflects their structure and outer body, which normally tries to protect the vehicle from the elements and the occasional collision, but in a role-playing game--player characters being player characters--it's inevitable that the question will come up: "How much damage will my Imperator assault rifle do to that guy's limo?" Vehicles thus receive a BAR value that helps determine the answer to just that question.
(Note: The addition of BAR flows back into standard BattleTech as well; there, a simplified version of the AP vs BAR rule essentially sums the above effects thusly: When a unit hits a target with any weapon that delivers more tactical armor damage than the target's BAR, the attacker gets to roll for an additional critical hit after applying damage, even if the target still has armor after that attack. What this means is that WorkMech with commercial-grade armor (BAR 5) could have 25 points of armor in its Center Torso, but if it gets hit by a BattleMech-scale large laser, the attack not only delivers 8 points of armor damage to the WorkMech; it also rolls for a critical hit, because the WorkMech's armor was effectively pierced.)
Hope that helps.
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