In the United States, "military-grade" is often more of a marketing or political term than a precise legal one. While you cannot generally walk into a store and buy a modern, fully automatic service rifle used on active battlefields today, you can legally purchase civilian versions and certain authentic surplus items. Defining "Military Grade"

The term usually implies equipment built to specific government standards (MIL-SPEC) for durability and reliability.

Civilians have several legal paths to owning firearms with military heritage or specifications:

Civilian-Legal Versions of Military-Only Guns - Pew Pew Tactical