This is a game of Infantry combat in the Battletech universe using Alpha Strike rules as the base.
The Alpha Strike rule mechanics work surprisingly well for this, and while we use 28mm figures, I would not change anything if you are using 15mm figures. Everything is abstract anyway, so just keep with the 6"/24"/42" combat ranges.
The core concepts are:
1) Infantry don't get much in the way of TMM, so how they use cover is more important.
2) Infantry don't have ablative armor plating, so critical hits are used to simulate the "nasty, brutish, and short" nature of infantry combat.
3) Each model has its own record sheet
Core Concept #1:
These are the new to-hit modifiers:
Target: Elemental Jumping +1
Target: Jump Infantry Jumping +2
Elemental in Cover +1
Infantry in Cover +2
Defensive Infantry in Cover +3
Self: Jumped this turn +2 shooting/+4 melee
Target is Defensive Infantry who did not move +1
Target: Large Vehicle -2[/li]
The "in cover" modifiers are not cumulative. You only get one based on your troop characteristics. Vehicles don't get any cover modifiers in this game system, as they are effectively in close assault with infantry. We are gaming super close range combat here, and infantry has the upper hand at these ranges. In cover modifiers also only count against shooting, not close assault (hand to hand).
You might notice right away that Defensive Infantry in cover is pretty much the only way to get infantry that is consistently hard to hit. They are either +3 to hit when moving into cover, and also have their 1 TMM, or they are stationary and "dug in" for a total of +4 to hit. These are intended to be the majority of trained infantry. Infantry without this trait are militia, conscripts, or have an aggressive, attacking doctrine that teaches them to be on the move for one reason or another. Solahma, Elementals, and Jump Infantry lack the Defensive trait, for instance. Probably all power armor should, a consequence of being that extra large compared to a normal infantryman.
Core Concept #2
Critical hits are made much more common against the relatively unprotected infantryman. While a mech has tons of ablative armor, most infantry isn't even fully armored! So stuff happens. The less protective gear that your infantry has, the more chance that they will be killed by a smaller amount of incoming fire.
We use the pilot die system, and it is important to use here for the mechanics. In this method of play you roll one die a different color than the rest, which is your "pilot die". You also roll as many dice as you have attack dice. So an LMG gunner with 3 damage value will roll 4 dice to hit, and one of them must be clearly understood to be the pilot die by the opponent. (We tend to use the kickstarter dice for the pilot dice). The pilot die is added to each attack die and the hit is determined for each. For example if you needed an "8" to hit, and you rolled a pilot die of "3", and your attack dice are "1, 4, 6", then the 3+1 and 3+4 combinations are misses. But the 3+6 is a hit, so your target takes 1 damage.
Any time the pilot die and the attack die are DOUBLES, and the dice combination is a hit, then also score a critical hit. (Can be multiple crits per attack, like if you roll four 6's)
Any melee attack hit from a Power armor unit also scores a critical hit (just one, even if does multiple points of damage)
Any SRM attack that hits will cause a critical hit on any infantry unit within 6 inches of the target model, even if it scores no damage (welcome to concussive blast effects!)
Any hit that is in "structure" will cause a critical, like normal
Infantry using improvised explosive devices against tanks or power armor will cause a critical hit to the target if they hit, and a critical hit to themselves, no matter what
Critical Chart:
Elemental/Power Armor Jump Infantry/armored Militia/no real attempt at protection
2 KIA KIA KIA
3 Stunned KIA KIA
4 No effect Jump Pack Destroyed KIA
5 Harjel breach Stunned Stunned
6 No effect No effect Stunned
7 No effect No effect No effect
8 No effect No effect Stunned
9 Harjel breach Stunned Stunned
10 Jump Pack Destroyed Jump Pack Destroyed KIA
11 Stunned KIA KIA
12 KIA KIA KIA
Harjel breach = Something knocked loose on Power armor, +1 to all future critical rolls, including this turn. Cumulative.
Stunned = lose moving and shooting next turn. No TMM
Jump Pack Destroyed = if you have one. Otherwise the contents of your backpack are destroyed. Likely only matters in a campaign situation.
SRM attack: Resolve as a direct fire weapon. Attack uses a single die + pilot die. If it is a miss, nothing further happens. If the target is hit, however, damage must be resolved. Vehicles take damage from direct hits only. They take 1 point of damage to their Alpha Strike profile, and a roll is made on the Mobility crit table just like in Alpha Strike. Infantry takes 2 potential damage if within 6 inches and not fully out of line of sight from the blast. Roll a new pilot die and 2 attack dice against each infantry target. Resolve any critical hits that may come up due to doubles AND every infantry that was a legitimate target for the blast (not out of line of sight) also takes a further critical hit for the concussive blast effect.
Melee attack: Resolve as per normal Alpha Strike rules. All infantry is size 1. All mechs are size 4. Vehicles cannot make melee attacks, just like Alpha Strike.
Elementals/Power Armor have 3 attack dice in melee
Infantry armed with Improvised Explosives may use them against mechs, tanks, and power armor, for a 2 dice attack, and will always generate a critical hit on themselves AND their target, in addition to other critical hits, or mobility crits, that may occur.
Infantry roll 1 die for regular melee with their bayonet/kukri/rifle butt.
Core Concept #3
Unit stats. I use the Master Unit List Custom Card Generator to make the stat sheets.
http://www.masterunitlist.info/Tools/CardGeneratorWeapon profiles are balanced around:
2 dice for a typical rifle. Some weapons can reach to long range, some cannot, some have a reduced damage profile at long.
2/2/2 = battle rifle OR small laser from a mech/tank
2/2/1 = fairly standard rifle profile
2/2/0 = short ranged weapon, carbine or high powered SMG. I use this for the Jump Infantry profile mainly
3/2/1 = Very rapid firing close ranged weapon. I use this for clan Solahma small arms
3/3/3 = SAW or LMG type weapon. Also a ".30 cal" type vehicle mounted weapon
4/4/4 = A ".50 caliber" type weapon Not man portable, although power armor might have this. Yikes.
For vehicles I have gone as far up as 6/6/6 for a Rifleman C using LB 5-X ACs with Cluster munition.
Vehicle versus vehicle shooting is simply resolved as an Alpha Strike attack with modified range bands. 0-24" is short range, 24-42" range is medium range, and 42" and beyond is long range. All normal Alpha Strike modifiers apply, including cover. Retain the original TMMs from Alpha Strike, no matter the modified movement values for Infantry Strike rules.
Some examples are listed as attachments. I gave LMG's and squad level Grenade launchers the option to use Suppressive fire. This is a 2" diameter template from a chosen point within LoS. Attack all models within that template with a 1 point attack, all models in that zone are also suffer a +2 to hit penalty when shooting next turn, and move at half speed next turn.
The Elementals are beasts. Low skill, incredible armor, great movement, great anti personnel weaponry, anti-mech melee attacks (they cause a critical), and a rack of the deadly SRMs.
Vehicles and mechs are actually super easy. Use their Alpha Strike cards like normal, except reduce the movement by half. If they shoot at infantry, you will need to use the Infantry Strike profile, but record the damage that they take like normal. I commonly use off table objectives and opponents for the on-table vehicles. Essentially they are fighting in a much larger battle, while also having to content with the up close and personal threats. One game an Uller was off map maneuvering to get a shot an Urban mech, which kept trying to hide from the Uller's side of the table, while also drawing a bead on the incoming Elementals, yet not get scragged by getting hit by too many SRM's from the Elementals all at once. This takes some creativity, but that sort of thing is far better than introducing mechs and vehicles and giving them nothing to contend with or do except what is on the table.
Some photos of games we have run so far:
A hetzer hides from a Rifleman, and is covered by an SRM armed Infantryman.
A scorpion has no better luck than the Hetzer
Solahma overrun a FedCom position
The Fuzz roll out to defend their turf in the Clan Invasion era Battle of Waldorf.
An "Urban mech" is covered by close infantry support as it moves around it's natural habitat