Gameplay Changes

List of Major Gameplay changes in comparison to vanilla

MASSIVE DIFFICULTY OPTIONS

The new game start and the ingame difficulty settings now include pretty much everything you can think of.

Reworked skill tree

Huge rework of skill tree to balance out utility and strength of each skill category.

Engine Cores have a number next to them referred to as Engine Rating. This number, divided by the Mech's tonnage, will give you the Mech's base walk speed in number of hexes covered. Ratings that are in between each "level" may improve sprint speed.

Engine Mods can provide a number of bonuses for trade offs. Most common are the Light engines (Light, XL, etc.), where extra torso space is traded for a lower engine weight.

Attack Improvement Mod

AIM is a BattleTech mod that fixes, enhances, and customize your combat experience, such as colored facing rings and targeting lines, tune down roll correction, show distance numbers, or a detailed attack log. The default settings aim to preserve the balance of vanilla game.

Fire Modes, Ammo Switching & Custom Ammo

There is now Custom Ammo supported, so HMGs, LMGs, MRMs, LBXs, RACs and other weapon systems now have their own ammo types. And with Custom Ammo, you can now use different ammo when firing your weapons. Switch between ammos with Left Click on a right side of weapon slot in combat HUD.

Heat Changes

Overheating will generate negative effects that worsen the longer you stay over the red line (42 Heat).

Heat Damage Soft Cap

For balance purposes Heat damage is soft capped. Due to the presence of MechLab, boating is quite prevalent and ruins the balance of missile based heat damage. Rather than nerf it into the ground, a stacking softcap has been introduced to discourage 'swarming' heat damage. Each weapon that deals heat damage reduces subsequent heat damage to the target by a small percentage until the mech's next activation. This is the effective heat reduction of up to 45 stacks of that effect. Currently, the equation is:

Heat Damage = Base Heat Damage * ( 1 - Percentage Reduction ) ^Stacks

Reworked Lance Matrix

Randomly generated lances have been rebalanced to work in harmony with value-based launch ratings. The overwhelming power of legendary units can mean very bad things at higher mission difficulties.

File:LanceMatrixTestingBuild.png

Combat game

TRUE RNG! Unlike in the base game, where hit chances are manipulated, no aim correction will occur. You either hit or you miss, the game doesn't hold your hand.
Melee has been buffed but Melee and DFA now have variable penalties for missing.
AI is now more aggressive and will try to outmaneuver you as a lance. Expect a lot of back shots into your 'Mechs if you give them the chance. They will also push their heat hard, and try to spike your own heat.
Morale is now affected by everything that happens during combat, even negatively, and Inspired grants a +2 accuracy at 75 morale. Morale is also capped at 150 in combat, max starting cap in combat is 100.
Vigilance is more expensive, up to 35 morale points per pilot.
Precision Strike is now Called Shot, allowing you to spend 10 Morale to do a called shot at higher (+6) difficulty. Your target will also move back 1 initiative phase.
Increased radar and vision range.
Moving incurs a flat accuracy penalty. Sprinting and jumping incur cumulative accuracy penalties (the more you move, the less accurate you are).
Can shoot after sprinting.
Evasion is more powerful, and is no longer removed when being shot at.
Accuracy and stability penalties for sustaining damage.
Elevation grants an escalating accuracy bonus and to hit penalty by 1 per 35 units.
Obstructed targets are harder to hit and incur a -10% damage penalty.
Indirect fire has a +8 to hit penalty and incurs a -20% damage penalty.
Increased maximum heat combined with reduced overheat thresholds.
Overheating incurs an escalating penalty to accuracy and movement as well as escalating chances to shutdown or having your ammo explode. The more consecutive turns you spend overheated, the higher the penalties and chance of shutting down or exploding.
Pilots can panic and eject! Continue pounding someone until they bail out or watch in horror as your legged ace gunner punches out after a barrage from the other side! Panic levels also have an effect on accuracy!

TAC - Through Armour Critical

Deal critical damage if your armour is compromised or if the weapon hits hard enough!
By standard TT rules, TAC was introduced to balance certain vehicles and 'mechs from being too robust, and make damage to still be able to affect them.
It's a real life addition taken to TT and RT, it wasn't added "just because", it's based on real physics too.
There is an in-game armour called Spall Liner which will lower the chance of TAC being done on your 'mechs (and OpFor too). Build smart.
Thickness defines the strength something can easily punch through, while shards defines how likely the hit causes spall to cause damage

=TAC Example=
Mech with 100 armor from 200 and full structure. 
Min crit chance 0.1. 
Weapon has APArmorShardsMod = 0.5 
APMaxArmorThickness = 150
APCritChance = 0.5

Shard Modifier = 1 + (1 - 100/200) = 1.5 
Thickness Modifier = 1 - 100/150 = 0.33333(3)

Overall chance  = 0.1 (Base minimum) * 1.5 (Shards Modifier) * 0.33333 (Thickness Modifier) * 0.5 (AP chance) = 0.025

As armor becomes lower, both modifiers for shard and thickness will rise.

Simgame

Starter 'Mechs are selected depending on your staring Faction.

Your starting lance is randomly assigned from between 210-240 tons and 3-6 'Mechs (including your starter choice, so choose wisely).

Playing on Company Strength, difficulty is determined by the strength of your Alpha team - the combined C-Bill value of your most expensive 'Mechs defines your available contracts. The number is determined by how many Mech's you are able to launch.

Your 'Mechs' value defines all aspects of their economy: the cost for maintenance, fueling, arming, and readying your 'Mechs all depends on the value of their chassis and equipment.

Having a high tech Gauss Rifle is harder to maintain than a crappy old Heavy Rifle, and fielding some Prototype 'Mech is costlier then fielding a mass-produced Centurion, for example.

Changing armour takes times and costs more.

You can use Shift+Click to change armor by a single digit each time.

Shift/Ctrl + Click allows you to sell 10/100 pieces of gear at once.

Assemble 'Mechs from pieces of similar chassis but expect them to be broken shot up pieces Yang slapped together. Keep only those you want and need, scrap pieces for money and sell your salvage.

Your monthly morale resets now to 10 plus your Argo bonuses, in order to create a greater incentive to invest into recreational facilities.

Your funding levels grant you a bonus or penalty to your Med and 'Mech tech levels - pay more and everyone works harder.

Pilots will be fatigued after a battle. While they can still be deployed, they will be operating with reduced skills and abilities until they recover. Also, pilot upgrades in the barracks won't be available until they return to active duty. Plan accordingly!

Skill Based Initiative

This mod makes the initiative order more random. Instead of 5 phases to each round, there are 30. At the start of each round, each unit's initiative in the phase is determined randomly. The basic roll will be between 6 to 12, though these bounds are influenced by several factors. Lighter mechs have a better bounds, while Assault mechs will have these bounds reduced. The Tactics and Piloting skill of the MechWarrior influence this range. Injuries, knockdowns, and various other battlefield conditions also play a part in determining the unit's place in the initiative order.

The practical effect of these changes is that novice Mechwarriors (low tactics or piloting skill) will typically act later in the round, even if they pilot light mechs. Highly advanced pilots have a better chance to act early in the round, even if they are in heavier Mechs. However the fickle hands of RNGesus plays a critical place, and you may find your well-placed battle plan undone by a set of poor initiative rolls!

This mod divides mechs into 10 ton categories instead of the light/medium/heavy/assault system. 20-25 ton mechs are slightly faster than 30-35 ton mechs. 80-85 tons mechs are faster than 90-95, which are faster than 100. Though randomness can hide this effect, the effect is there, offering a small bonus to pilots of lighter units.

Battlefield conditions that impact your place in the initiative order include:

Pilots with injuries suffer an initiative penalty between -4 to -7. High Guts ratings reduce this penalty to a minimum of -1 to -4. Pilots with additional health pips (from high guts ratings) will ignore one injury per additional health pip before feeling these effects.
Mechs that are knocked down suffer an initiative penalty equal to their Guts injury bounds in the turn they are knocked down. On subsequent turns they suffer a -6 penalty, reduced by 5% for each point of piloting skill (rounded down). A mechwarrior with piloting skill 6 reduces this penalty by 30%, to -6 * 0.70 = -4.
Mechs with a missing leg or vehicles with a missing side count as crippled and suffer an initiative penalty of -13. This penalty is reduced 5% for each point of piloting (rounded down). A mechwarrior with skill 5 reduces this penalty by 25% to -13 * 0.75 = 9.
Mechwarriors with the High Spirits tag gain a +2 initiative bonus. Mechwarriors with the Low Spirits tag suffer a -2 initiative penalty.
Mechwarriors that are Inspired by Morale or Fury gain a bonus between +1 to +3.
Any chassis or component that modifies the BaseInititiative of the unit should be honored. This value is normalized against the expected values for the weight classes (light=2, medium=3, heavy=4, assault=5).
Units that are attacked in melee suffer an initiative penalty. For every 5 tons of difference between the attacker and target, the target suffers a -1 init malus if the attacker is heavier. If the target is heavier, they will only suffer a -1 penalty. Units with the Juggernaught skill (AbilityDefGu5) will multiply this malus by a small amount. This penalty is cumulative so a target that's repeatedly attacked by multiple heavy units can be reduced to the lowest initiative level. This penalty is reduced by the 0.05% for each point of Guts the pilot has.
Units that reserve lose a random amount of initiative, between 2 and 7 phases.

Mech Sensor Icons

Cosmetic that adds floating icons near the Mech that point out the big targeting buffs and debuffs.

How do targeting, visuals, and sensors work? My brain hurts.

Mine too. Low Visibility is the mod that that provides the changes and you can head over to that page for a longer summary. In a nutshell, chance to hit relies on the big 3 concepts (detailed guides linked):

Accuracy modifiers. Can come from weapons, terrain, etc. Fairly similar to HBSBT, but RT just piles on more stuff to affect this.
Electronic Warfare that covers Sensors, ECM, Stealth, and more. No Sensors is a debuff that results in a large -7 chance to hit. Enough stacking ECM jamming can result in absolutely pathetic chances to hit. Like the time 4 command bunkers with their turret-boosted Angel ECMs spawn almost on top of each other and your entire lance took 4 rounds just to kill the first one. </salt>
Visibility, or how well you pilot can see what they're shooting at. Fog of war is not directly representative of Pilot's actual Line of Sight vision, and both are different from a weapon's Line of Fire. No Visuals is a debuff that results in a large -5 chance to hit. Yes, it's possible for an enemy Mech to John Cena your sorry pilot.

There is a Known Issue where auras (ECM/ECCM/etc.) are only calculated after movement, and the accuracy numbers in the weapon panel during movement preview are likely wrong.

Lance Spawner

Lance spawner adds a layer of RNG behind the skull rating, causing missions to vary slightly in difficulty depending on lances spawned.

Company Difficulty Special Challenge

The Company Difficulty setting combined with Lance Spawner, can push difficulty all the way up to a theoretical designed maximum equivalent to 12.5 Skulls. In development testing, the equivalent of 10.5 Skulls seems to be the maximum observed.