This is a stupid thing to attempt as BattleTech is woefully absurd. I would suggest you not take it too seriously.
Using the PD and ED stats for Metal Armor and modifying it slightly to compensate for a higher technology, ED having 15% value than PD.
BA is Battle Armor and BM is BattleMech.
PD ED BODY HERO Mech WPN 120mm Cannon
BA, Light 0.401-0.750 tons 8 9 6 9 (Destroyed) 5 (Crippled)
BA, Medium 0.751-1 ton 9 10 7 9 (Destroyed) 5 (Crippled)
BA, Heavy 1.001-1.5 tons 10 12 8 8 (Destroyed) 4 (Half)
BA, Assault 1.501-2 tons 11 13 10 7 (70% Damage) 4 (Less Than Half)
BM, Ultralight 10-15 tons 12 14 12 7 (Over Half Damage) 4 (One Third)
BM, Light 20-35 tons 14 16 14 6 (Less Than Half) 3 (~One Fifth)
BM, Medium 40-55 tons 16 18 16 5 (Less Than Third) 3 (One Sixth)
BM, Heavy 60-75 tons 18 21 18 3 (One Sixth) 2 (One Ninth)
BM, Assault 80-100 tons 20 23 20 2 (One Tenth) 2 (One Tenth)
Here's the bit that seems silly but is working out so far. To reflect The greater BAR rating of BT armor I decided to make all BT vehicles/BA/BM with 75% Damage Reduction to PD and all BT era weapons Irreducible.
Example of Large Mech Weapon (4d6AP vs ED) listed in HERO rules and M1A1 Abrams 120mm Main Cannon (8d6 vs PD) are also listed above.
As Modern Era Weapons do a fraction of listed damage to BT Era Armor... I'm still working on Damage Conversion. Some weapons are Physical and some Energy. (listed weapons are all BT era)
BT Damage PD ED
MG (Light); LRM; BA: King David Light Gauss Rifle; BA: MG (Light) 1 2d6+1 1d6+1AP
MG; SRM; BA: Magshot Gauss Rifle; BA: MG; BA: Suppoer PPC; BA: Plasma Rifle 2 3d6 2d6AP
MG (Heavy); Laser (Small); BA: MG (Heavy) 3 3d6+1 2d6+1AP
Laser (Medium); PPC (Light); BA: Laser (Medium) 5 3.5d6
Pulse Laser (Medium); BA: Pulse Laser (Medium) 6 4d6-1
Laser (Large); Gauss Rifle (Light) 8 4d4
Pulse Laser (Large) 9 4d6+1
PPC 10 4d6+1
PPC (Heavy); Gauss Rifle 15
Autocannon/20 20
Gauss Rifle (Improved Heavy) 22
HERO Listed Spacecraft Main Gun 10d6AP
Lot of guess work and would appreciate help. All Laser Weapons seem to be AP in HERO stats.
Lastly...
Pts. of Armor/DEF: BA 0-5=10; BA 6-10=11; BA 11-15=12, 16-44=12; 45-88=13; 89-132=14; 133-176=15; 177-220=16; 221-264=17; 265 or more=18
Tonnage/BODY: BA=10; 20,25=12; 30,35=13; 40,45=14; 50,55=15; 60,65=16; 70,75=17; 80,85=18; 90-95=19; 100=20
I'm out.
