All of this sounds really interesting, I wasn't aware of all these small arms nor the HMMVW's story!
Something else I found is that choosing the GM XM-1 over the Chrysler one has actually much more impact that I thought:
-I can't find this document anymore but in a congressionnal hearing on the delays and cost overruns of the XM-1 programm, it was mentionned that not choosing the GM tank would have very negative consequences for several already weak states, such as Michigan. Many people such as those in the Teledyne company that made the AVCR-1360 engine would suddenly have no work, and the chairman explaining the problem made it seem like the SecDef's decision to choose Chrysler was really screwing over states and people that didn't need it.
One can wonder if saving Chrysler this way didn't actually have more negative impact on the economy than if he had chosen GM. The latter option might have better balanced the situation, at least for the time being.
- It seems like choosing GM would be quite a big deal regarding delays and additionnal costs. Outside of the fact that the decision to choose one design was delayed for a good time (IIRC several months), you also had to seriously rework the lackluster Chrysler prototype (change the turret, FCS...) and fix issues with the turbine, which is more expensive than the diesel. Choosing the GM in July 1976 would have likely accelerated delivery by over 8 months and prevented $1B from being used, and this was only a low-end estimate by the Congress. The AVCR-1360 engine is a well-proven, mature design taken from the MBT-70 and as such required pretty much no extra developement, unlike the turbine. The turret is also pretty much what the real Abrams ended up using so not many changes would have been required.
Overall if it's compared to the more sophisticated contemporary Leopard 2 I think that getting the Abrams in service in 1979 if not earlier is definitely possible.
- Without the delays, cost overruns, the costly turbine and all the massive logistics required by it, the GM Abrams would likely be much cheaper, and also more attractive due to it's lower price and more conventionnal design.
- The tank not only outperformed the Chrysler prototype in trials but it seems that it would have had quite a few improvements over a vanilla Abrams: it is slighly smaller, lighter, it is possible to shift gears manually (which is a plus for skilled drivers), the tank had hydropneumatic suspension elements on 3 roadwheels per side (which leads to even lighter weight and less volume taken inside the tank as well as better performance), and what's more is that such a suspension configuration was also tested in the M60A1E3 so if the GM Abrams has them it might butterfly an hybrid suspension on the M60A3 thanks to commonality.