All they have is the projectile, or bullet? (or have they located the spent brass, too?)
I'm wondering if it isn't some sort of handloader projectile, which is unique enough to identify the manufacturer. Otherwise...would NATO projectiles be as easy to identify as NATO brass is? I don't know. Not familiar enough with 5.56 NATO rounds to tell the difference. More into the 7.62, .308 basic equivilant. I don't know if one could tell for sure about those either, unless they were 146 grn. boattails, or something. Otherwise, they all look the same to me. So --- what could it be? Slightly different bases? Metallurgical signature? Saw it crosswise and look at how the lead "ball" is, and how thick or thin the case walls are? What makes it ID as U.S. manufacture?
Just tossing this out there...since I'd bet some freeper might know.
Ok, read the article at the link [after posting, in fine freeper fashion]. Lake City Arsenal, huh? Birthplace of my .308
ball ammo boattails. They work real good in an old re-worked Mauser I used to be fond of. But then that gun used to really like 180 grn Federal “Classic”. ‘Can’t seem to find that particular ammo anymore...but it was amazingly good in that old rifle. There were times I could pick off a golf ball at a hundred yrds with set-up, using iron sights, to boot. Don’t quite have the eyes for it, now.