A4 refers to the AR-15/M16A4 style upper receiver, which features a flat top with a Picatinny rail for mounting optics or detachable rear sights, instead of the fixed carry handle found on A2 models. The A4 upper works with AR-15 lower receivers, standard barrels, bolt carrier groups, charging handles, handguards, gas tubes, and front sight bases or gas blocks. It’s ideal for modern builds, offering versatility for mounting scopes, red dots, or backup iron sights on AR-15 platforms.
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