Peter

In fact, you shouldn't use a flash unless you have diffusers etc to bounce the light off.

The Best way to take photos of a gun is Outside on a
Bright or very Bright but CLOUDY day.

This gives you the light you need to capture the detail but not a direct source of lightt will shine / reflect off the metalwork.

Try it sometime in Direct Sunlight and then the same photo when the sun has gone behid a cloud.