[swaghauler]

The US Army has no proper "field guns." A field gun was a WW2 term for a towed Anti-Tank gun. the Russian 37mm, 57mm and 76mm WW2 Anti Tank guns were "field guns." The US embraced the concept of a "general utility platform" for its artillery after WW2. This was based on observations about the German use of the 88mm Flak gun as an AT gun, an AA gun, and a fire support weapon. The Army later dropped any AA missions from the field artillery.
The US Army did continue the concept of a multipurpose gun with regards to AT and fire support. Both the M102 105mm GUN/HOWITZER and The M198 155mm GUN/HOWITZER that I served on with the 10th Mountain had HEAT warheads and the M198 could fire the Copperhead laser guided AT missile. Both the HEAT and The Copperhead could be fired indirectly if needed. You would fire the HEAT at an extremely steep angle to induce plunging fire (for bunkers) and the Copperhead would stabilize itself in flight. Mortar crews were just our little brothers. We were even slated to begin using the same fuses in our rounds (they were converting over to ours). While AT missions were part of our training; I would not have wanted to do it in real life. Our guns had manual traverse and elevation. Our 155mm shells weighed 96 lbs and we had a maximum effective range of 18km with conventional shells. The newer "base bleed" shells (which generated a gas vortex behind them) were good for about 22km (depending on the shell- HE, GAS, HEAT, etc..) our enhanced RAP rounds (rocket assisted projectile) were good for almost 40km but our Circular Probability of Error (CEP) increased significantly. The CEP of conventional shells was about 100 meters around intended impact point. Newer low drag and base bleed rounds brought that down to 30 meters CEP at 15Km and about 50 meters CEP at maximum IFR.

It is important to note that the rounds a 105mm, 155mm, or 203mm Howitzer fired travel at just under 900 meters a second. it would take TEN SECONDS for a round to travel 9km. The IFR rules in Twilight2000 should take this into account.

In my game it takes 1 minute to call a fire mission plus 2 rounds to fire the gun. I then add the appropriate flight time. My players will often "make a stand" to give the FO time to "call fire." The FO also is required to have a Topo Map and a compass in order to call fire (he must determine the enemy's grid coordinates). My players made the FO the navigator as well.

When determining hit location of rounds; I use 1d20 - FO's roll under skill/hit chance X 5m for deviation of the rounds (0 means an "ATTABOY!, or direct hit- these are much coveted in the artillery) with a CEP of 100m. I use 1D6- FO's roll under his skill/hit chance X 5m for rounds with a CEP of 30m. On a miss; I add the amount missed by to the CEP roll.
I also divide the Indirect Fire Range into 4 range bands which add (12km IFR becomes SHT-3km, Med-6km, Lng-9km, Ext-12km) with appropriately decreasing to hit skill to represent the difficulty of long range fire.

I hope this helps you out.

Swag