Thread: What to make of weird chrony data?

Interesting timing with cbfb's query re. chronograph reliability, but I'm just using what I've got which is a ProChrono Digital...

I finally got a chance to test some 243 loads I'd loaded ages ago. Fouled the freshly-cleaned rifle with my last three PowerShoks, and got predictable results of around 3130 FPS (wasn't taking much notice as I was only checking zero / fouling the barrel). Then I started testing my hand loads and got some results that were all over the place. Any ideas what I should make of this?

41.4 grains: 2969 - 2969 - 2969 (1.0" group) Chrony seemed to be working fine, just giving the exact same speed each time. Oh well, it's only the starting load...
41.6 grains: 2956 - 2956 - 2949 (2.5" group) Hmmm... unusual... group could have been shooter error, two very close and the third 2.5" higher.
41.8 grains: 2917 - 2956 - 2982 (1.5" group) Scratching my head now...
42.0 grains: 2956 - 2904 - 2930 (1.75" group) Magic number again?! But getting slower - WTF?! Bolt lift getting sticky...
42.2 grains: 3016 - 2989 - 2904 (3.5" group) Not expecting any sort of accuracy with that velocity spread, and bolt lift quite sticky now.

Rifle is nothing flash, a Mossberg ATR, factory-standard apart from a bedded action. Shooting prone off a bipod.
Projectiles are Targex 95 grains with 20 thou jump.
All cases were the same head-stamp, shoulder-bumped and trimmed to 2.035".
Powder is W760, cos that's what I could get. I had trouble finding a starting load, ended up extrapolating some info from Berger and Hodgdon.
Conditions were average - overcast & a bit windy, and the chrony was only about a metre from the muzzle due to the topography.

Velocities seem pretty low compared to the factory PowerShoks and what I thought I should expect. Apart from the sticky bolt lift, there were no other pressure signs, and this rifle seems to get a sticky bolt lift with factory ammo. So where to from here? Play around with seating depths at 41.5 grains? Keep going higher and see if there's a node further up the line? Re-test in better conditions with the chrony further away? Try a different chrony?

For comparison, the same chronograph and powder gave much more consistent results on the same day in my 7mmWSM, albeit also on the slow-for-charge side (averaging 2915 FPS with 60.0 grains behind a 150 grain ABLRs).