Originally Posted by
MistWolf
Blend the damage smooth, the refinish the worked area. The ideal thing would be if you could treat it with Alodine then hit it with some paint. If the fracturing is localized, you'll be ok. If there is micro fracturing over a larger area, you could have problems. My guess, from looking at the photos, the fracturing is localized. 7075 forgings are pretty tough. But that's a guess. It should be fine to continue shooting but just keep an eye on it. If the fracturing continues, get a new upper receiver. (I deal with aluminum aircraft structures including forgings as a professional technician.) After blending but before applying Alodine or paint, examine the area with a strong light and at least a 10x magnifying glass for cracks. You can also use dye pen. Dye pen is probably over kill, but I'd rather advise you to use it and have you find nothing than tell you everything will be fine only to discover the damage is more extensive than first thought.
Blending will reduce the stress risers the damage has introduced. Blend it just deep enough to smooth out the damage and long and wide enough so there is a smooth transition to the undamaged area. Don't go crazy and don't over think it. By hand, blend with 180 grit emery cloth and water and maybe follow it up with 220 grit. You can tape off around the damage to control the size of the blended area