I'm basing many of these factors off the rules in A Time of War.
For example, a weapon with an AP of <4 can't do any damage, at all, to a mech, ever, period. An AP of 5 can, on a very good roll, do minor damage to the canopy which has a BAR of 9 instead of 10. Only weapons AP 6+ have any real chance to hurt armored targets (mech/tanks/etc with BAR 10). Even an AP 6 weapon has -4 damage vs an armored target.
Lets take a standard Auto-Pistol. Its AP is 3, and damage is 4. This means it has ZERO chance to hurt a mech, so I would give it a hard attack value of 0. Its soft attack value would be .4, as each grunt takes about 10 damage, and 4 damage / 10 = .4 per weapon.
An Automatic Rifle has AP 4, and 4 damage, which means it does the same damage against infantry, but against armored targets it could do AP 4/10 = 40%, or .16 damage to an armored target. I am leaning towards half that damage if the AP is exactly 4, as they have to aim for the cockpit. In that case the hard attack of the rifle would be .08, or a 28 man platoon would do all of 2.24 damage to a mech! Right now they'd do like 15 damage!
Now lets take some anti-mech weapons, lets say its a clan ultra-heavy support laser, the most power anti-mech weapon infantry can carry. Its AP is 7, and its damage is 14. It would do 1.4 damage to infantry per weapon, but against armor it would do 7/10, or 70% of 1.4, which would be .98 damage per weapon. If a platoon had 4 of these, 1 per squad, it would do 4 damage to mechs, a heavy platoon with 2 per squad could do 8.
We can assume the ranges given in ATOW are against infantry, as that last weapon has a range of 63 BT hexes, but only 15 in BT. We can assume that difference is due to battlefield conditions. Lets face it, rifles maximum effective range is rarely actually reached in combat, and ranges at which the rifles get used are often much less.
Another difference is each MoS in ATOW is +1 damage, or +10% damage based on the way I'm calculating damage. Since I'm multiplying damage by the AP/10, the ultra heavy clan support laser with an MoS of 7 would do 2 damage points for a single weapon, instead of 1, however 8 of them would only need, on average, an MoS of 1-2 to gain +1 damage point. I think the minimal increases in most situations, it is acceptable to ignore this damage increase weapons would get in the RPG.
Doing the half AT damage if AP is 4, here are the Soft/Hard (or Anti-Infantry/Anti-Tank) values for all the weapons in ATOW. I also gave them more specific range bands based on their range in ATOW, with the maximum being the Clan ER Heavy Support Laser at 16 hexes (basically all range bands were divided by 150m). This gives the weapons a bit more flavor, and easier to calculate range brackets.
Soft/Hard (SR/MR/LR/ER) = Weapon Name
0.4/0 (0/0/0/0) = Auto-Pistol
0.3/0.06 (0/0/0/0) = Auto-Pistol (Hawk Eagle)
0.4/0 (0/0/0/0) = Auto-Pistol (M&G Service Auto)
0.5/0 (0/0/0/0) = Auto-Pistol (Magnum)
0.2/0 (0/0/0/0) = Auto-Pistol (Mydron)
0.4/0 (0/0/0/0) = Auto-Pistol (Nambu)
0.3/0 (0/0/0/0) = Auto-Pistol (Serrek 7875D)
0.4/0.08 (0/0/0/0) = Auto-Pistol (Sternsacht Python)
0.2/0 (0/0/0/0) = Auto-Pistol (Stetta)
0.6/0.3 (0/0/1/2) = Elephant Gun
0.4/0 (0/0/0/1) = Federated-Barrett M42B (Close-In Mode)
0.5/0.1 (0/0/1/2) = Federated-Barrett M42B (Standard Mode)
0.4/0.08 (0/0/0/0) = Harpoon Gun (Pequod, Mk. 2)
0.3/0 (0/0/0/0) = Harpoon Gun(Pequod, Mk. 1)
0.3/0 (0/0/0/0) = Machine Pistol (Martial Eagle)
0.3/0 (0/0/0/0) = Pistol (Hold-Out)
0.4/0 (0/0/0/0) = Pistol (Makeshift)
0.6/0 (0/0/0/0) = Pistol (Sternsnacht Claymore)
0.4/0.08 (0/0/0/0) = Revolver
0.5/0.1 (0/0/0/0) = Revolver (Magnum)
0.4/0.08 (0/0/1/2) = Rifle (Automatic)
0.4/0.08 (0/0/1/3) = Rifle (Bolt-Action)
0.5/0.1 (0/0/1/3) = Rifle (Federated Long)
0.4/0.08 (0/0/1/2) = Rifle (Imperator AX-22 Assault)
0.4/0.08 (0/0/1/3) = Rifle (M&G G-150)
0.4/0.08 (0/0/0/2) = Rifle (Makeshift)
0.4/0.08 (0/0/1/2) = Rifle (TK Assault)
0.5/0.25 (0/0/1/2) = Rifle (Zeus Heavy)
0.3/0 (0/0/0/0) = Semi-Auto (TK Enforcer)
0.3/0.06 (0/0/0/0) = SMG (Gunther MP-20)
0.2/0.04 (0/0/0/0) = SMG (Imperator 2894A1)
0.2/0.04 (0/0/0/0) = SMG (KA-23 Subgun)
0.2/0 (0/0/0/0) = SMG (Rorynex RM-3/XXI)
0.2/0 (0/0/0/0) = SMG (Rugan)
0.4/0.2 (0/1/2/4) = Sniper Rifle
0.4/0.2 (0/1/2/4) = Sniper Rifle (Minolta 9000)
0.3/0 (0/0/0/0) = Submachine Gun
0.3/0.15 (0/0/0/1) = Blazer Pistol
0.4/0.2 (0/1/3/7) = Blazer Rifle
0.2/0 (0/1/4/8) = Ebony Assault Rifle (Extended-Range Mode)
0.4/0.2 (0/1/2/4) = Ebony Assault Rifle (High-Power Mode)
0.3/0.06 (0/1/3/6) = Ebony Assault Rifle (Standard Mode)
0.3/0.06 (0/0/0/2) = ER Laser Pistol
0.4/0.08 (0/2/4/9) = ER Laser Rifle
0.2/0.04 (0/0/0/0) = Holdout Laser Pistol
0.3/0.06 (0/1/3/8) = Intek Laser Rifle
0.3/0.06 (0/0/0/1) = Laser Pistol
0.3/0.06 (0/0/0/1) = Laser Pistol (White Dwarf)
0.4/0.08 (0/1/3/7) = Laser Rifle
0.3/0.15 (0/1/3/7) = Laser Rifle (Maxell PL-10)
0.4/0.08 (0/1/3/7) = M61A Combat System
0.5/0 (0/1/2/6) = Magna Laser Rifle
0.3/0.15 (0/1/3/7) = Marx XX Laser Rifle
0.4/0.08 (0/1/2/4) = Mauser 1200 LSS
0.3/0 (0/1/2/4) = Mauser 960 Assault System
0.5/0.1 (0/2/4/9) = Mauser IIC IAS
0.2/0.1 (0/0/0/1) = Nakjima Hand Laser
0.5/0 (0/0/0/0) = Nova Laser Pistol
0.2/0 (0/0/0/1) = Pulse Laser Pistol
0.3/0 (0/0/0/1) = Pulse Laser Pistol, Clan
0.3/0 (0/0/1/3) = Pulse Laser Rifle
0.4/0 (0/1/2/5) = Pulse Laser Rifle, Clan
0.4/0.08 (0/1/4/8) = Starfire Er Laser Rifle
0.4/0.08 (0/0/0/1) = Sunbeam Laser Pistol
0.3/0.15 (0/1/3/8) = Dragonsbane Pulse Laser
0.7/0 (0/0/0/0) = Flamer, Heavy
0.5/0 (0/0/0/0) = Flamer, Man-Pack
1.2/0.72 (1/5/9/16) = Heavy Support Laser, ER (Clan)
1/0.6 (1/4/8/15) = Heavy Support Laser, ER (Non-Clan)
1/0.6 (0/2/5/10) = Heavy Support Laser, Semi-Portable (Clan)
0.6/0.36 (0/1/3/7) = Hellbore Assault Laser
0.8/0.16 (0/1/3/8) = Particle Cannon, Semi-Portable
1.4/0.7 (0/1/4/9) = Particle Cannon, Support
1/0.5 (0/1/3/6) = Plasma Rifle, Man-Portable
0.8/0.48 (0/2/5/10) = Support Laser
1/0.6 (0/3/6/11) = Support Laser, ER (Clan)
0.8/0.4 (0/2/5/10) = Support Laser, ER (Clan, Semi-Portable)
0.8/0.48 (0/2/5/11) = Support Laser, ER (Non-Clan)
1.2/0.84 (1/3/6/12) = Support Laser, Heavy
0.7/0.35 (0/2/4/9) = Support Laser, Semi-Portable
0.5/0.25 (0/2/4/10) = Support Pulse Laser
0.5/0.3 (0/2/5/11) = Support Pulse Laser, Heavy
0.4/0.2 (0/1/4/8) = Support Pulse Laser, Semi-Portable
0/0 (1/3/6/12) = TAG, Light (Man-Portable)
1.4/0.98 (1/3/6/12) = Ultra-Heavy Support Laser (Clan)
0.5/0 (0/0/0/0) = Automatic Shotgun
0.6/0 (0/0/0/0) = Avenger CCW (Buckshot)
0.6/0.3 (0/0/0/0) = Avenger CCW (Solid)
0.5/0 (0/0/0/0) = Combat Shotgun
0.6/0 (0/0/0/0) = Double-Barreled Shotgun
0.6/0 (0/0/0/0) = Double-Barreled Shotgun (Sawed Off)
0.4/0 (0/0/0/0) = M&G Flechette Pistol
0.4/0 (0/0/0/0) = M&G Flechette Rifle
0.5/0 (0/0/0/0) = Needler Pistol
0.5/0 (0/0/0/0) = Needler Pistol (Hold-Out)
0.4/0 (0/0/0/0) = Needler Pistol (Sea Eagle)
0.5/0 (0/0/0/0) = Needler Rifle
0.6/0 (0/0/0/0) = Pump Shotgun
0.6/0 (0/0/0/0) = Pump Shotgun (Sawed-Off)
0.5/0 (0/0/0/0) = Shredder Heavy Needler Rifle
0.5/0.1 (0/0/0/1) = Gauss Pistol
0.4/0 (0/0/0/0) = Gauss Pistol (Mandrake)
0.6/0.3 (0/0/2/5) = Gauss Rifle (Thunderstroke II)
0.6/0.3 (0/0/1/4) = Gauss Rifle (Thunderstroke)
0.4/0.2 (0/0/1/4) = Gauss Submachine Gun
0.4/0.08 (0/0/0/1) = Coventry Handrocket
0.6/0.12 (0/0/2/4) = Gyrojet Gun, Heavy
0.4/0 (0/0/0/1) = Gyrojet Pistol
0.4/0 (0/0/0/0) = Gyrojet Pistol, Holdout
0.5/0.1 (0/0/1/3) = Gyrojet Rifle
0.4/0.08 (0/0/0/1) = Gyroslug Carbine
0.5/0.1 (0/0/0/1) = Gyroslug Carbine (Star King)
0.5/0.1 (0/0/1/2) = Gyroslug Rifle
0.4/0 (0/0/0/0) = Buccaneer Gel Gun
0.5/0 (0/0/0/0) = Ceres Arms Crowdbuster
0.3/0 (0/0/0/0) = Dart Gun
0.2/0 (0/0/0/0) = Flare Pistol
0/0 (0/0/0/0) = Gas Capsule Pistol (“Spitballâ€)
0/0 (0/0/0/0) = LGB-46R “Paint†Gun
0.5/0.1 (0/2/5/10) = Radium Sniper Rifle
0.4/0 (0/0/0/0) = Sonic Stunner
0.4/0 (0/0/0/0) = Tranq Gun
0.6/0.3 (0/1/2/4) = Mk 1 Light AA Weapon
0/0 (0/2/6/12) = Mk 2 Man-Portable AA Weapon
0.1/0 (0/0/0/0) = Medusa Whip
0.3/0 (0/0/0/0) = Mini Stunstick
0.5/0.1 (0/0/0/0) = Monowire
0.5/0 (0/0/0/0) = Neural Lash
0.5/0 (0/0/0/0) = Neural Whip
0.4/0 (0/0/0/0) = Stun-Staff, Double-End
0.4/0 (0/0/0/0) = Stun-Staff, Single-End
0.4/0 (0/0/0/0) = Stunstick
0.4/0.24 (0/0/0/0) = Vibroaxe
0.2/0.12 (0/0/0/0) = Vibroblade/Vibrodagger
0.3/0.18 (0/0/0/0) = Vibrokatana
0.5/0 (0/0/0/0) = Vibromace
0.3/0.18 (0/0/0/0) = Vibrosword
0.3/0.21 (0/0/0/0) = Vibrosword, Clan
0.5/0.25 (0/2/4/10) = David Light Gauss Rifle
0.6/0.36 (0/0/2/6) = Grand Mauler Gauss Cannon
0.5/0.3 (0/2/5/10) = King David Light Gauss Rifle
0.7/0.42 (0/1/4/8) = Magshot Gauss Rifle
0.6/0.36 (0/0/2/7) = Tsunami Heavy Gauss Rifle
0.7/0.49 (0/0/0/1) = Autocannon, Bearhunter
0.3/0.18 (0/0/1/3) = Autocannon, Semi-Portable
0.5/0.1 (0/0/1/3) = Federated-Barrett M42B (LMG Mode)
0.3/0.15 (0/0/1/3) = Machine Gun, Light
0.4/0.2 (0/0/1/3) = Machine Gun, Portable
0.4/0.2 (0/0/1/3) = Machine Gun, Semi-Portable
0.5/0.25 (0/0/1/4) = Machine Gun, Support
0.7/0 (0/0/0/0) = Support Needler, Firedrake
Ord B (0/0/0/1) = Grenade Launcher
Ord B (0/0/0/1) = Grenade Launcher, Automatic
Ord A (0/0/0/1) = Grenade Launcher, Compact
Ord C (0/0/1/1) = Grenade Launcher, Heavy
Ord D (0/0/1/1) = Grenade Launcher, Heavy Automatic
Ord D (0/1/2/3) = LAW
Ord C (0/2/6/14) = LRM Launcher
Ord D (0/2/7/10) = MRM Launcher
Ord E (0/1/2/3) = SRM Launcher (Heavy)
Ord E (0/1/1/3) = SRM Launcher (Light)
Ord E (0/1/2/4) = SRM Launcher (Standard)
Ord C (0/1/1/3) = VLAW
Ord E (0/1/2/6) = Mortar, Heavy
Ord D (0/0/1/3) = Mortar, Light
Ord E (0/1/3/4) = Recoilless Rifle, Heavy
Ord C (0/1/2/4) = Recoilless Rifle, Light
Ord D (0/1/2/4) = Recoilless Rifle, Medium
Ordnance based weapons, such as Mortars, LAW/VLAW, Recoilless Rifles, SRM/LRM, and Grenade Launchers have damage based on their ordnance values:
A = 0.8/0.00 AP, 0.6/.12 HE
B = 1.0/0.00 AP, 0.8/.40 HE, .3/0 Inferno
C = 1.2/0.24 AP, 1.0/.60 HE, .5/0 Inferno, 1.0/0.00 Stun, 1.0/.80 Anti-Vehicle
D = 1.3/0.26 AP, 1.1/.66 HE, .7/0 Inferno, 0.5/0.25 Flash, 1.1/.88 Anti-Vehicle, 1.0/0.50 Air-Burst/Guided
E = 1.4/0.28 AP, 1.2/.72 HE, .9/0 Inferno, 0.6/0.42 Flash, 1.2/.96 Anti-Vehicle, 1.1/0.55 Air Burst/Guided
Gas is available for all, and is always 0.5/0.0. C/D/E can use Narc. Any can use smoke/Flare, D/E can use FASCAM.
Using these tables, instead of those in TM, make the weapons more true to ATOW, make infantry less lethal and less "munchy", and are very easy to implement.
The downside is infantry armor isn't modeled very well now, and it would still be kinda silly. I also haven't yet figured out how to implement burst fire weapons. In ATOW you fire a burst, take a modifier to hit, and for each MoS you can get up to the size of the burst in extra shots. I think maybe just a table for "burst strength", that shows you additional infantry damage per MoS or something, but using a burst goes through ammo (and you get maybe 1-4 bursts before being out and not shooting for a turn, or needing a reload).
Another method, which would require more math while in play, would be totally accurate to ATOW, but I doubt very well liked, is this. Instead of tracking Hard/Soft damage, just keep track of AP/Damage, though you'll have to do it for both the primary and support weapons (and any disposable ones). These numbers would be the same as they are in ATOW. You would then have a table that compared the damage vs BAR for each weapon, and you would simply do that much damage. This would be great against battle armor, which rarely have a BAR of 10, as infantry would be more effective against them. It would also allow much more accurate damage against armored infantry. This would be my recommended way of playing, if you only had a few platoons on board, or wanted to do a battletroops style game, but for anything very large it would probably be too cumbersome.
Another note, right now damage for some weapons (M42B) are excessive due to their integral grenade launchers. As these weapons have limited ammo, and aren't fired all the time, and most assuredly aren't fired at the same time as the rifle, I recommend these systems be treated as additional disposable weapons, which can be fired *instead* of the normal attack, and may or may not have a limited number of shots (modern grenadiers can have a dozen to 30 rounds on them).