Hornady 160 grain RN not stable

[mikehill85]

I am working on a subsonic load for a Tikka Super Varmint in 6.5 Creedmoor with a 1:8 twist rate. I was able to work up a subsonic load for it but the rounds don't seem to be stable. The Berger stability calculator shows the round should be stable. At 25 yards, the bullets seemed stable but at 100 yards, it's obvious that they aren't. I'm just wondering if anyone knows why it isn't stable even though the stability calculator says it should be. I realize that the Berger website states that the calculator isn't accurate for flat base bullets. Does anyone know of a similar calculation for flat base bullets? I am just trying to figure out if there might be another reason the bullets aren't stable (e.g. going though the transonic region).

P.S. I realize there isn't any practical reason to develop a subsonic 6.5 Creedmoor load. I am just doing it for fun.

Bullet impacts at 100 yards

Berger Stability Calculation

 

[Macht]

That stability calculator isn't really meant for subs. There's also a difference between 'stable with noticeable yaw' and tumbling.