Re Abrams's Rheinmetall 120mm vs. AC/5:
"Modern" tank guns in our time are in BT rules "rifles" (even though MBT guns are almost all smoothbore nowadays). Rules are Experimental/Lvl3, in Tactical Operations, but here is a short version:
All rifles subtract 3 from their damage points when attacking any battlefield unit except conventional infantry, battle armor, 'Mechs with commercial armor, and support vehicles with a BAR less than 8.
Light rifle (<100 mm): dmg 3 (0 against 'Mechs), ammo per ton 18 (ludicrously heavy, but it's BT for you)
Medium rifle (100-125mm, or MBT main gun late 20th-early 21st century): dmg 6 (3), ammo per ton 9
Heavy rifle (155-203+mm, heavy artillery or "future" tank cannons): dmg 9 (6), ammo per ton 6
Then there's the matter of Barrier Armor Rating or BAR. All of "our" armour has nothing but "BAR" in BT terms, ie. it's all or nothing - not even reactive armour would be enough to act like BT "ablative armour" giving you x points per facing. IIRC 22nd century tech would give you BAR6 but our tech would be more like BAR5 or even BAR4. What this means is that if a weapon's Damage is greater than BAR, you need to test for criticals even if the shot wouldn't damge internals (but since you don't have "real" armour, it would also damage the internals). A medium laser or AC5 would thus automatically penetrate Abrams, damage internals and cause crits. All 31st century 'Mechs and tanks have BAR of 10, plus they can take advantage of the regular armor rules. (Of course, if you play Lvl3 you should test for crits also in 'Mech-to-'Mech combat, if you hit with weapons that cause 11+ damage.)
