Barrel Rifling

Found something odd the other day.
Me and a buddy were out at the lake doing some plinking and we set up a few water melons. I unfortunately forgot to pack the hollow points so I had to use my standard FMJ target rounds which of course was very anticlimactic and just punched holes in em.
Behind our target range is a large hill that consists mainly of sandy soil this being in East Texas,and while checking out the less than stellar damage to the melon I found a slug that was totally complete with almost no malformation to it. In fact checking it with a micrometer showed no damage to the circumference and it varied from .440 to .448 depending on whether you were checking across a land area left by the rifling or not but it is consistent all the way around the slug.
And here's the odd part....there are six land areas on the rifling of a FNX .45 barrel but it it only left 5 grooves on the slug. The 5 grooves are evenly spaced and than there's a gap between one and five where the 6 groove should be.
Looking through the barrel everything looks just fine and you can easily see all 6 land areas,yet one didnt transfer to the slug.:evil:

I guess I'm going to have to put some rounds into a water tank and see if I can recreate my findings.

Any thoughts?