Okay, so I am fairly certain everyone has had the moment where they thought or even said aloud "I really wish I could fire into that assault!". The 40K universe is a dark place, full of ruthless adversaries and sometimes a general is required to be just as ruthless to achieve victory. The following rules are the product of a few fellow gamers combining their ideas and opinions. The result, I believe, is a simple and easy house rule that represents friendly fire.

All normal 40K shooting rules apply if they are not specifically countered here.

Normal non-template weapon shooting units in assault:

To fire into assault first you choose your target unit and roll to hit and wound that unit as normal. Then the controlling player of your target unit receives a 4+ cover save against your shooting. The saves are rolled as normal, however if the targeted unit has a better armor save then 4+ they are able to use it, as long as they would normally be able to do so due to AP of weapon fired etc. All successfully saved wounds from the cover save of 4+ are then inflicted to other units in the assault by the targeted units' controller. Please note that only the wounds saved on 4+ from the cover save are transferred; saves from a greater armor save are simply negated as they normally would in shooting.

Example 1

5 Ork Boyz and 5 space marines are locked in assault. It is the Ork player's shooting phase and he chooses to shoot into the assault with another Boyz squad before joining the assault. The 20 Boyz roll to hit and wound resulting in 6 wounds (Yes, very lucky shooting I am aware! This is just an example) The Space Marines now need to roll to save against these wounds. Being in assault, they receive a 4+ cover save and their armor save is 3+. The Space Marine player rolls 6 dice and the result is 1-six 2-fours 2-threes and 1-one. The space marines have made 3 of the 4+ cover saves and 2 additional armor saves. The Space Marine squad suffers 1 wound.

Now the Orks in the assault must save against the 3 wounds that the space marine player saved against with the 4+ cover save granted to them by their own Ork flesh. They use their armor saves as normal and fail all 3 suffering 3 wounds.

Shooting into assault with template weapons

Place the template marker and roll for scatter if required as normal for the weapon. When the position of the template is determined count the number of models x(x=friendly + non-friendly) under the template. The shooter then rolls to wound their target squad using x dice as the number of hits. The saves are rolled as normal, however if the targeted unit has a better armor save then 4+ they are able to use it as long as they would normally be able to do so due to AP of weapon fired etc. All successfully saved wounds from the cover save of 4+ are then inflicted to other units in the assault by the targeted units controller. Please note that only the wounds saved on 4+ from the cover save are transferred saves from a greater armor save are simply negated as they normally would in shooting.

Example 2

An imperial guard infantry squad manages to survive an assault from 10 Ork Boyz with the sergeant and 2 guardsmen remaining. It is now the IG shooting phase and the IG player wishes to fire a grenade launcher into the assault. He places the template and rolls for scatter as normal. Under the template are 4 Ork and 2 guardsmen a total of 6 models. The IG player rolls to wound 6 Orks and scores 4 wounds.

The Orks are in assault and receive a 4+ cover save which is better then their 6+ armor save. 2 of the dice are 4+ resulting in the Orks suffering 2 wounds.

Now the IG squad must save against the 2 wounds inflicted upon them by granting the Orks a cover save. They roll their armor saves as normal failing one and suffering a single wound.