Artillery (what sort?) in Cham armies

[tadamson]

Duncan has given you the core info..
hui hui pao predominately refers to counterweight trebuchet, but was also used as a generic term for non gunpowder artillery (aka catapult in western references).  These may have reached Champa via China, or through Muslem merchants.
Champa was a trading nation with naval forces, so would have ship born weapons as well (unlikely to be trebuchet).

There are some reports in China from 'envoys', some of whome were military. These are very generic and don't go into units and tactics.

logistics would have been mostly two wheeled ox/buffalo carts, elephants were more akin to 'engineers'.

Don't entirely dismiss chariots. They have their place as prestiege transport, however again generic terminology often  simply means 'wheeled' and is later translated as 'chariot'.

There was also a thriving trade in books during this period so Chinese, Indian, Muslim, and other information was available in the region.

[Aetius-last-of-the-Romans]

Don't entirely dismiss chariots. They have their place as prestiege transport, however again generic terminology often  simply means 'wheeled' and is later translated as 'chariot'.

I totally agree that there are 'chariots' or two wheeled carts, as you suggest, primarily as 'prestige' transport, not as weapons of war.
Chou Ta-Huan refers to them accompanying the King in parades at Angkor. One does appear at Banteay Chhmer, with a general riding it in a parade, and they also appear at Bayon - where I believe they are being used to transport the elephant howdahs.

This latter use may not seem as implausible as it sounds (e.g. "surely you'd get the elephant to carry its own howdah?") but later British colonial discourses on the use of elephants suggests that:

a). Elephants easily injured their backs if carrying any heavy loads on them over an extended period of time (& howdahs are not that light). NB: an elephants strength is in its shoulders, head and truck - it can lift huge weights with its tusks and trunk, but is limited to a maximum of c.25% of its body weight on its back, but only for a limited length of time. This might also explain the depiction of the Khmer King (or a very senior member of the Royal family) riding horses, accompanied by mounted fan and parasol bearers, on the march.

b). when let loose to forage the armies elephants were unloaded.

c). when moving through dense forest most loads were removed from the elephants and carried on carts.

One of the 'chariots' at Bayon is described by Michele  Jacq-Hergouale'h (in his The Armies of Angkor book) as carrying weaponry for its absent warrior. However, my own view, based upon viewing that carving and my photographs is that it could just as easily be a 'dismounted' elephant howdah carried in a cart. Which might explain its slightly odd representation.

Mark