M1, M1IP, XM1.
Next, in chronological order, with game-related changes:
A base-model M1 with the 120mm M256 cannon and some weight simulator plates. Plates were plain steel, so no armor increase.
An M1IP with a 120mm M256 gun.
Unmanned-turret prototype M1 with 120mm M256 gun and 44-round 12 rounds/min autoloader. 3 crew seated in the front, so no fuel tanks flanking the driver.
A heavily modified standard M1 fitted with XM291 120/140mm gun and autoloader. Tank could hold 17 140 or 120mm ready rounds in a bustle magazine, with 22 140 or 30 120mm rounds in a hull magazine. Autoloader could load at between 8-12 rounds/min.
Sequel to the above with a new turret, still containing the XM291 120mm/140mm gun and autoloader. Still looking for more information on this one.
M1A1 with depleted uranium armor package in the turret only. Significant improvement vs. KE shells.
Heavily modified testbed model. Engine was replaced by 1450 HP advanced diesel, gun by 120mm XM291 and autoloader as above (140mm was never tested on vehicle), transmission had 7 forward gears instead of 4, hydropneumatic suspension used instead of torsion bars, modular armor and multispectral smoke on turret, and 'signature reduction measures' (read as dust/radar reducing cowboy fringes on side skirts).
M1A1HA (with applique)
An M1A1HA with 1 inch high-hardness steel plate welded to the turret front as a field modification during Operation Desert Shield.
Marine corps variant of the M1A1 with 2nd-gen DU armor in the turret only. Same armor as base-model M1A2.
M1A1 with new commander's sight and 2nd-generation DU armor in the turret only.
M1A2 with revised armor package for export to Saudi Arabia.
M1A2 with 3rd-generation DU armor in the turret only- though 5 tanks had DU in the hull as well.
Export model M1A1 for Iraq. Similar to M1A1SA (for Morocco) and M1A1 AIM SA (for Australia) in that it had a different armor package from contemporary US M1A1s.
USMC M1A1 with M1A2 SEP's 3rd-generation DU armor package.
M1A2 SEP v2
M1A2 SEP with commander's remote weapons station and ERA on hull and turret sides. TUSK upgrade can add another M2 machine gun in a coaxial mount over the main gun.
There's more, of course, but these are the interesting ones.
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