Above posts pretty much cover it. Just a little more clarification.
For Operation Desert Storm, a.k.a Gulf War, 1990-91.
US Army used M109A5, M109A6 Paladin was not fielded till after the war, about 1994. Italeri did make a kit of the A5 as well, you could actually build any version of M109 with the kit. It was listed as an M109A2/A4 kit, same vehicle on exterior. I think it is now discontinued with the intro of Paladin kit. It does pop up quite often on Ebay though. And no, you cannot backdate a Paladin to an M109A3 without lots, and lots of work. It has a totally new and diferent turret, along with some other items.
M109A5
M109A6 Paladin
Also used were the afore mentioned M102A1 105mm howitzer, available from AFV Club.
And the M198 155 towed Howitzer that is available from Mouse House Designs from Australia for a small fortune. http://www.mheaust.com.au/MHE/Resin/OZA/M198.htm
M198
Lastly, the big guns used are the M270 MLRS available from DML.
M270
USMC used both the M198 and also the M110A2 8" howitzer (203mm.), available from Italeri as well.
USMC M110A2
Some USMC units still used the old WWII era 105mm M101A1 howitzer as well, avialable from Italeri, may be out of production though.
M101A1
For Operation Iraqi Freedom, Iraq 2003-present.
US Army is using M109A6 Paladin, M119A1 105mm howitzer, M198 155mm howitzer, and M270 MLRS. Some Army National Guard and Reserve units are still deploying with M102A1s as well
M119
USMC is still using M198 exclusively. They have retired all other artillery pieces and rely on the US Army for additional fire support.
Hope that clears it up some more for you.
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