Nick Grant was asking so I thought I would post my personal modifications for Arty Confliffe's Crossfire to make it more adaptable for 21st Century games. (and late 20th century).
It's important to note that I've gotten alot of inspiration and the core of my ideas from Matakishi's Tea House (in my links section). Matakishi is a huge Crossfire enthusiast and his works are both inspiring and a little genius. I recommend anyone go there and check out his Counter-insurgency ops page.
I keep the regular mechanics of Crossfire the same as I like the system and feel it is a great representation of modern infantry combat.
Instead of playing at the Squad level, the basic unit of maneuver is a Fireteam (one half of a squad). What's nice about this is now you can play Crossfire in 1:1 for many Western Armies as a fireteam is roughly 3 to 4 troops. So 1 Fireteam is 4 dudes who I like to have bunched together or at about 1" apart to represent their cohesion (actually 1 base width since we don't use measurements or distances in Crossfire). Players will note, as in Matakishi's changes, that now a platoon is roughly the size of a company when you use Fireteams as your basic unit of maneuver. (3 squads times 2 fireteams is 6 manuever units, plus 2 weapons sections and command elements = 9 total units)
Summary: Unit = Fireteam
Western Fireteams with M249 Minimis (US Soldiers affectionately call it a SAW or Squad Automatic Weapon) and Under barrel grenade launchers get 4 attack dice.
Summary: Western Fireteams get 4 attack dice, Insurgent Fireteams get 3 dice, unless they have a light machine gun like an RPK, PKM, etc. Then they get 4 dice.
Forming Crossfires:: The WWII Rules for forming crossfires still apply, however if there is radio communications between fireteams, then the Squad Leader or Platoon Leader attempting the Crossfire makes a roll. Veteran troops roll a 3+ on a 1D6 to form a Crossfire. If successful they get to fire, if not successful only the fire team in contact shoots. Line of sight to the PL or Squad Leader is not necessary although LOS to the target obviously still is.
Summary: Make a leadership check (same dice you'd use to rally from Pinned) to determine if the Squad Leader or Platoon Leader can form a Crossfire when elements do not have LOS to the leader stand.
(the leadership mods still apply, and also Western armies equipped with PAK-4 or other IR Lasers add 1 to the roll)
IED Attacks: IEDs attack in almost the same way as Artillery however prior to the game opening players should agree to the nature of the IED and also the trigger mechanism. (Cell phone, trigger man w/ wire, pressure plate, etc) Also the size of the IED and nature of it as well (Artillery shells? metal gas tanks filled with explosives? Or more sophisticated EFP weapons)
RPGs / Vehicles: Someday I'll publish the values I use for Armor however suffice to say it's constantly changing. For instance the other day I used a +/- 0 To Hit modifier for the RPGs and a penetration of only 1. The Bradley's Armor was a 3 which arguably should have been higher thanks to the application of bolt-on shaped charge defeating Armor systems. None of the RPGs that hit the Brad penetrated, which I think is fairly realistic.
ROE: A very important aspect of modern, counterinsurgency operations. Requesting permission to engage and staying within the Rules of Engagement. For instance I can tell you in Baghdad in 2004, tankers had to seek permission from their Battalion Commander to use their main guns in the city (Obviously post-invasion). However coaxial and .50 calibers were authorized. As the war progressed, I imagine that became even more restrictive. After 2005 in Iraq, you needed a General Officer's permission to use 155mm Artillery in many cases!
I represent this by the following rule: Vehicles such as Bradleys receive 5 fire dice which ignore all but the toughest cover modifiers, however they must seek permission to use the main gun. Same with main battle tanks as well. So if you want to use the on-board MGs that's fair game, but using heavy, vehicle mounted weapons requires a successful roll of 4+ on a 1D6.
These are my modifications. Crossfire is so unique that it can host a huge amount of small adaptations and still keep the flavor of fast-moving and fluid infantry combat alive and well. So there are any number of ways you can add modifications and still keep the system intact.
Here is a wrap-up:
Size: 4 troops, roughly 1 base-length apart
Leadership: Squad leaders and Platoon Leader/Platoon Sergeant
Crossfires: Can be formed via radio on the same roll it takes your troops to recover from Pinned.
Insurgent Teams: Regularly get 3 dice but add a 4th 1D6 if they have an LMG
ROE: Large vehicles need permission to use their main-guns in all situations - same with Attack Aviation.
IEDs: Ensure everyone agrees to the size, scope, type, and firing mechanism.
IED attack / Placement:
Crossfire moves around so much that pre-plotting IEDs might not be the way you want to go. We like to use IEDs as an insurgent bonus attack, to be used whenever the insurgent player wants, wherever he wants. Another key reason why the amount should be agreed upon before the game starts. He will need a successful "trigger man" roll to kick it off though, something he could not accomplish last game.
remember too that if you don't have the necessary number of figures, you can always adjust as well. In my games i use 3 US troops for a fireteam and 2 insurgent figures for an insurgent fireteam. in this way i still have it aesthetically pleasing with a decent ratio but it's not critical to play.
Anyways, these are my humble ideas for Crossfire. There are many adaptations and mine are by no means the best. I just wanted an easy and quick way to use Arty Conliffe's Crossfire rules in a modern setting and to recreate some of the experiences of modern combat, especially in Iraq. If you disagree with any of my findings or methodology, that is perfectly ok with me! Please feel free to change any way you see fit. That being said, if you have a better idea, definitely post it in the comments section as I would be happy to modify my rules for an idea that makes the game better. All of us is smarter than some of us or one of us!