You may have one of those super fast 270s.
I've been accused of having the fastest 270 Win out there. Been shooting it since 1966. She's on her second barrel.
In mine, 28.5gr RL-22 pushes the 140 NAB right at 3200 FPS according to the Chrony Beta.
I don't trust the Chrony much. Only use it for ES and SD calcs on promising loads. I then go long range and watch for vertical dispersion.
I use the 0.496 bc and adjust velocity to get the drops correct.
Each rifle will produce a different bc for any given bullet. I don't suppose the difference is all that large but is observable as distances increase.
The main objective is to hit the point of aim and I have better luck using the published bc and adjusting velocity.
To get a 0.6+ bc with the 140 NAB and 150 BT I had to repoint the bullet with a brass tip which also increased the weight almost 10 grains.
55.5 grains of PP 4000 MC causes the chrony to read 3225. I'll validate that velocity as soon as the wind quits.
7828ssc gets the old girl well over 3300 fps mv which sure seems a bit much for the winny. However, barrel and case life has got to suffer. Thus I backed down to the PP 4000 MC velocity which produced exceptional groups.