If you upgrade your LR with a pintle-mounted storm bolter would that then count as a secondary weapon system for psycannon bolt eligibility? If so, would purchasing psy-bolts once for an LR apply to both the p-msb and heavy bolter or would you have to purchase the bolts separately for each weapon?

If I remember correctly, it would count for both. And upon getting and reading my codex, it does. For some reason only storm-bolters and heavy bolters (twin-linked or not) can use psy-cannon bolts, aside from the psycannon itself.

Unfortunately though no psy-bolts for the LRC Hurricane bolters nor for foot-sloggin' heavy bolters. Imagine the extra carnage that could of been done to a Daemon army. Wonder why the foot bolter can upgrade but the vehicle can't? Vice versa for the heavy bolter as well. Makes no sense.