There was a general problem with Stryker hull height and cargo bay height in C-130. The MGS version already exceeds C-130 dimensions IIRC, and lifting the vehicle with C-130J was the rationale behind using a <20 ton vehicle. A turret was thus not acceptable. The handful of MGS is an exception and can get the privilege of C-17-only airlift.

They could mount some lightweight autocannon such as 20 mm M621 or 30 mm ASP-30 on a mount like those used for .50cals, though.