What are these markings on found 0.50 BMG Bullets?

A friend of mine has found hundreds of these 0.50 BMG bullets while out metal detecting near Lake Havasu and Mohave Valley in Arizona. A bunch of them have these crosshatched markings on one side of them and we can’t figure what they are. Any ideas about what could have created them? They’re not on every bullet and they aren’t consistent bullet to bullet.

He found them near the FUDS Yucca aerial gunnery practice site shown in the photos. Any ideas?