More thoughts regarding the Trigg becoming Austin scenario.
According to the OR, Hardee's "Corps" on 26 May 1862 comprised four brigades, one of which, Cleburne's, included Clarkson's (Calvert's) and Trigg's Batteries (OR Series I, Vol. 10, Pt.2, pp.550). On 30 June the "corps" showed a fifth brigade, Hawthorn's, made up of regiments that had been drawn from the other brigades of the "corps" and included Austin's Battery (OR Series I, Vol.10, Pt.1, p.788). On the same return, Cleburne's brigade only contained Calvert's Battery. It seems reasonable to assume that Austin's Battery had previously been Trigg's and had been transferred to Hawthorn's brigade when that unit was created. Presumably Hawthorn's brigade was broken up at the end of July 1862, Austin's Battery being disbanded as previously discussed.
As Austin's CSR records show him to be a member of Calvert's Battery, it may be that he was assigned to Trigg's Battery on a temporary basis as commander (what happened to Trigg?) and when the battery was disbanded he was transferred back to Calvert's Battery.