Just got back from shooting my first 1:7 twist upper using XM855PD.  While I was shooting there were a few times I heard something I had never heard before; I shoot BANG and then a sssshhhh.  At first I thought I was hitting a rock, but  there werent many rocks around where I was shooting.  It almost sounded as if a person were to shoot a shotgun parallel to the ground through tall grass.
This only happened maybe 10-15 times out of about 300 rounds.

Any thoughts or am I going crazy?

I don't see how a 62 grn. projectile could spinn apart from a 1/7, were other people shooting around you?  An ajacent pistol range?  Sounds like riccocet pistol rounds off the berm spinning and falling in your firing lane maybe.

I had one a few months back while walking back from down range, heard a buzzing sound like it was comming twards me and then a puff of dirt a few feet away.  I was on the rifle lane closest to the pistol range.

I dont see how a 62gr would come apart either, but you may be right as there were other people nearby shooting.  Next time I go out I'll be sure to get to a secluded area and see what happens.  The more I think about it the less I think it was a round spinning apart, because I could always see a definite dirt cloud where I had shot.
Thanks for the reply

Nope... I was running a "mutt" round... the kind left over from a range session and is of uncertain parantage.

The shot patterned.

I'm gonna guess it was a Hornady bullet in a Federal load.

It came apart... I had no clue until the shot hit.