On my quest to make the perfect Twilight 2000 Sheridan I think I've finally found it.
Believe it or not but way back in 1967 there was already concerns that the M81 (as it was then) Gun/Launcher may not be a good idea. As such Rock Island Arsenal did a crash program of four other weapon systems to fit in the M551 turret should the M81 develop problems.
Of the four only two were recommended mainly due to space problems, although it was mentioned that if serious redesign work was done all four would fit in the turret. The two weapon systems were:
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M32 76mm Cannon, the same as was on the M41 Walker Bulldog
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XM180 105mm Gun/Howitzer from the
XM104 super mobile lightweight howitzer (which really should have gone into service)
Of the two the XM180 was the preferred weapon system as it coupled low pressure and trunnion loading with high damage output and ammunition that was still largely in service.
This weapon fired much faster than the existing M81 because it didn't need a compressed air purge to blow out the bore so the combustible cartridge cases wouldn't ignite prematurely. It's likely that by the time of the Twilight War an A1 version of the gun/howitzer would have been developed with a bore evacuator for even faster firing. Notably the XM108 could fire any 105mm howitzer ammunition in US stocks and new racks for the vehicle gave a stowage of 50 Rounds. (I note the UK ammo has a squash head round)
If the M551A1 is the M81-armed standard version with vision upgrades and a minor modernisation package that would make the M32 76mm the M551A2A1 and the XM108 105mm the M551A3A1
Here's an image of the gun way back in 1967
