The ejector spring assy can cause this. The spring can be to long or the angle of the tip is wrong. Some say this is normal for this but it can be fixed by trimming one coil off the spring at a time and then test firing to see if it goes away. Its called M14 hickes.
If you got a GI stock then a dummy selector kit could also stop the brass from dinging the stock.
After I put a kit on my GI birch stock the brass hits the connector arm and not the stock. Got my kits from Hawk because his come with the connector arm.
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