Could be digital scale issue. ALWAYS weigh an item of known weight on your scale to check calibration/accuracy before each loading session. Takes one second and eliminates one variable in the puzzle...I have a Browning X-Bolt in 28 Nosler I've been shooting for a couple of years with 143 gr Hammer Hunters. I recently got my hands on some ADG brass and N570 powder. I've been wanting to try some load workup with these new components. Went to the range a week ago to do pressure test and it went like this:
83 grains - 3465 fps with no pressures signs
84 grains - 3575 fps with no pressure signs
85 grains - 3600 fps with no pressure signs
86 grains - 3722 fps with slight ejector mark, no sticky bolt
Decided to focus around the 86 grain level (+/- .3 grains) to test accuracy.
Back at the range yesterday to try those loads and they shot .5 MOA BUT they all ran velocity around 3500 fps instead of around 3700.
I've been loading for 35 years and never seen anything like this. Components were exactly the same each time:
New ADG brass
143 grain Hammer Hunter bullet
using Magnetospeed chrono
Anybody experienced anything like this?? It's driving me crazy trying to figure out why velocity would drop 200 fps with exact same components, seating depth, etc.