To clarify for the OP, these don’t generally scale down the whole gun. Caliber conversions can be as simple as swapping the bolt.
You can also sometimes swap to a larger caliber. They make ‘conversion kits’ for the AR-15 that are functionally just most of a new gun. You can get those for a variety of calibers from 7.62x39 to .50 Beowulf.
To clarify for the OP, these don’t generally scale down the whole gun. Caliber conversions can be as simple as swapping the bolt.
You can also sometimes swap to a larger caliber. They make ‘conversion kits’ for the AR-15 that are functionally just most of a new gun. You can get those for a variety of calibers from 7.62x39 to .50 Beowulf.