Today we delve into an older title: The cold war platoon level rules No End in Sight https://www.wargamevault.com/product/135451/No-End-in-Sight-Cold-war-and-modern-platoon-combat 

No End in Sight (NEIS for short) does not have a standard "declare that this guy is now on reaction fire" approach. Instead any soldier that is not pinned down is on the look out and can conduct reaction fire.

When a unit is moving across ground that is in sight of un-pinned enemies, it happens by way of the Rush move. 

Let me illustrate using state of the art combat simulation graphics

We have here three Blue soldiers behind some bushes, who would dearly like to get over behind the corner of that building. 

The empty ground between the two locations is being observed by two Reds though. 

Its 3" to reach cover. So we roll 1D6 for each soldier Rushing and that is how far they can go.

Better for some than for others!

The 5s make it all the way across. The guy rolling a 2 however moves the 2 inches he rolled, then becomes pinned down and must roll to see if he got hit.