Factory ballistics are "corrected to the muzzle". This means that the velocity is raised a little from what you would read on a chronograph. Generally the difference is small. 10 or 20 fps.
GPM
Printable View
its coming out of a remington 783
Could just be a massive chamber and freebore.....tho 600fps is a hell of a lot.
Been a while since I crohnyed any factory ammo but I remember doing 308 fed blue box threw my standard Sako and being surprised it was within 50-60 fps of what the box said.
I was surprised at the time because of the wildly held beleif that the box fps is very optimistic....in that case it was pretty bloody close I thought considering the 22ish inch barrel of the Sako