House Rules are as below, these are a work in progress. All published errata from MSL are in play.
General:
1 SIM = 1 meter by 1 meter
We will be using a 1 inch square grid since hexes get wonky with this rule set.
Diagonal movement costs double for every other square of movement.
The optional rule regarding reduced cost for training is in play. A "hard use" will be awarded by the GM for succeeding at a skill check requiring a result of 5 or more over the average of the dice pool. The GM may at his discretion award a "hard use" for crowning moments of awesome or creative skill uses.
Ascension grants toxin resistance, note that this is not radiation resistance. Keep this in mind if your dosimeter turns red or the geiger counter starts clicking. I can assure you it's probably more than a banana.
Gear repairs are based on structural technologies for anything without moving parts, mechanical technologies for mechanical items and electronics for anything with integrated circuits. Items based on vacuum tubes would fall under electronics as well for sake of simplicity.
The DR for repairing an item in the field is repair multiplier X damage intended to repair. You may choose to repair less than the full amount of damage, however you only get one attempt to repair an item in the field. If time is critical, assume 1 minute X repair multiplier per point of SI repaired. Example: Light armor with 20 points of SI damage would be a DR of 20 to repair fully. While the same amount of damage to medium armor would be a DR of 40. The repair times would be 20 and 40 minutes respectively.
Repair kits are as follows: Armor Repair kit, 3 kg, 200T, able to repair up to 100 SI of damage. Electronics repair kit, 2 kg, 400T, no SI limit. General mechanic's kit, 6 kg, 300 T able to repair up to 500 SI. Gunsmith's kit, 2 kg, 200T, able to repair up to 100 SI.
Repairs to items if a full shop is available are assumed to succeed. Cost is SI x repair multiplier Trust and 1 hour for every 10 points (or fraction) of SI being repaired.
Hero Points are awarded 1 per 250 total CDP (bonus CDP from species do not count). A point may be used to add 1d8 to any roll, a result of 8 is a critical success while a result of 1 is a critical failure. Hero Points may optionally be permanently burned for certain results, such as "Hand of God". 1 Hero Point will refresh between sessions, ALL Hero Points refresh between plot arcs and points may be refreshed at GM discretion for crowning moments of awesome.
Combat:
Stance changes can only be performed once per active segment. This is to prevent the munchy "I sprint then go prone to shoot and then change back to sprinting to keep my DR up" shenanigans that RAW implies as legal.
Laser dot sights and optical/reflex sights do in fact stack in the 5 SIM range they share.
Rifles do additional damage in order to balance them with SMGs. The largest damage die increases one step, thus a 7mm rifle doing 3d4+1 would do 1d6,2d4+1.
Grenades are a range rating 1 weapon when thrown. Base DR is 8 within nominal range and maximum range is based on the throwing character's strength. A grenade is assumed to weigh .5 kg. Determine maximum range by dividing 10% of the character's managable weight by 2 until the quotient is less than the weight of the grenade. Take the number of divisions and double the base of 10 meters for each one (effectively 2^x). Example: a str 20 soldier wants to throw a grenade, he can throw 10% of his manageable weight (54 kg) 10 meters per RAW. A grenade is ~ 10% of that so he could throw it up to 80 meters (5.4/2 = 2.7/2 = 1.35/2 = .675. 10m x2^3 = 80). At nominal range (up to 10 meters) his DR is 8, at maximum range (80 - 10 = 70 SIM past nominal) his DR is 78 to hit a 1 SIM square. Drift will be calculated by rolling 1d8 to determine direction and then drifting the margin of failure SIM in that direction. If our sample soldier throws a grenade 20 meters at a 1 SIM window his DR would be 18, assuming a skill proficiency level of 3 from core fitness his roll would be 2d6,4d4 + 3 (assuming he dedicated all of his dice for the cycle). If he rolls 18 or greater the grenade hits the window, which shatters assuming he managed to do more than 3 damage to it (1d4 for mass + 2 for strength). Assuming he rolls his minimum of 9, he would then roll a d8 and the grenade would drift by 9 SIM.
Launched grenades follow the above grenade rules, excepting that launchers are range rating 4.
Chunky salsa is in play assuming a sturdy enough barrier.
Endurance may be used for "breath control" while shooting at a 2 to 1 ratio. Up to 3 points at a time may be used this way. The maximum bonus would be a +6 using this rule. This is in addition to the other uses of endurance per RAW. This rule is intended to give ranged specialists something to use endurance for other than a buffer against bleeding.
Ranged called shots to a vital area against an unsuspecting target are per a normal called shot. SPL is added as a damage modifier. This requires a FULL cycle of aiming in order to make the shot. A trained spotter cuts this time in half (3 segments). It still costs at least 1 END for a ranged called shot.
Wielding:
Time on Target optional rule is in play. Minimum wielding time is 1 segment.
Wielding effects are obvious and visible to the naked eye, usually effects manifest as a glow around the wielder's hands.
Wielders must be standing still and have both hands free in order to wield.
DR for concentration checks is an average resist (DR 15) plus 1 for each point of damage received. Failing a concentration check results in the effect failing. The endurance cost for the effect is still expended as normal.
Gear:
Ammunition prices are as RAW when in populous areas. Common pistol and rifle rounds cost 50% more in the wilds, uncommon are doubled as are rare rounds. Rare rounds may not be readily available outside of major population centers.
Regular and high yield frag grenade prices are reversed as printed in the book.
Weapons which include melee attachments may be purchased without the melee attachment. The base price is reduced by the cost of the attachment.
In order for magazines to be available in combat for reloading, they need to be stowed in an accessible pouch. Small pouches can hold 1 STANAG (rifle magazine) or two SMG/pistol magazines. Large pouches hold 2 STANAGs or four SMG/pistol magazines.
Utility harnesses are assumed to have enough PALS attachment space for reasonable amounts of gear. They include optional drop-leg panels.
Backpacks are assumed to have quick releases and can be dropped in combat. Just be aware that if you retreat, that your backpack may be lost.
You may have weapons rechambered in a different caliber. This will be accompanied with a change in magazine capacity and damage. Minimum cost for this will be 25% of the weapon's value for a factory option or at least 50% of the weapon's value for a gunsmith to do it after market.
Pistol class weapons have space for one rail mount attachment under the barrel and one on top of the barrel.
SMG have space for one rail mount attachment under the barrel and one on top of the barrel. SMGs are compatible with slings and are assumed to have attachment points for them.
Assault rifle have space for four rail mount attachments around the barrel and one on top of the receiver. Assault rifles are compatible with slings and are assumed to have attachment points for them.
Pump Shotguns have space for one rail mount attachment under the barrel and one on top of the action. Shotguns are compatible with slings and are assumed to have attachment points for them.
Auto Shotguns have space for one rail mount attachment under the barrel, one on top of the action and one additonal on the non extraction port side of the action. Shotguns are compatible with slings and are assumed to have attachment points for them.
Support weapons have space for four rail mount attachments around the barrel and one on top of the receiver. Support weapons are compatible with slings and are assumed to have attachment points for them. Support weapons may be mounted to tripods and pintle mounts, in order to do so any attachments on the bottom rails must be removed.
Mortars, tube weapons etc. do not have rail mounting space. You are not allowed to go mall ninja on a mortar.
Bows can mount optics, please consult with the GM for total available space.
edit 05/18: grammar and grenade mechanics, I screwed up my math.
edit 5/19: added new gear repair kits
edit 5/26: Hero Point rules added
edit: 6/6: fixing typos, new "sniper" rule