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).

Bolt lift sticky with the 42 grain load, why shoot the 42 .2 ? you know its over the limit already, what did that proove to you ?
Higher velocity doesnt equal better group.

So the start load had a low ( none) spread for ES and SD but a good group.... just stick with that.
There is an old saying
Chronographs have spoiled more groups than ladder tests.

Reshoot the 41.4 and 41.6 before you do anything such consistent velocities indicate a node.

In my experience, once you start seeing erratic velocities you are passed having stableoafs and likely over pressure for that particular load combination in that rifle. Back it off.

Personally I would happily be sticking with 41.4gr load.

What weight projectile were the powershoks? I'm guessing lighter than the Targex

I'd be a bit dubious about your first three rounds, seems a little bit too consistent, could be the crono only registered the fist round... Just a thought. There's some serious spread in the last load, it could of course be dodgy crono readings but something else to have a look at is consistent neck tension on your reloads, I've found a looser neck will give dramatically slower velocity.
Just another something to consider.

chrony was only about a metre from the muzzle due to the topography

I'm surprised at that distance the muzzle blast didn't disturb the chronograph, even with a medium capacity round.
And muzzle blast can affect the readings. You've obviously got a placemat issue as per your posting but it would be much better if the chrony was 4 metres out.
You can use a ballistic program to calculate muzzle velocity.

Optical chrographs are such finicky devices.
Your first string of 3 exactly the same velocities would have me questioning the chrony.

Some would also suggest you should (must?) use magnum primers with the W760.
Although having said that my count using W760 is around 600 shots fired and I never have had a misfire or erratic velocities due to suspect ignition.

If you should try magnum primers just be careful as you may get an increase of velocity/pressure.

I have had issues with the chrony being too close to the muzzle and the blast causing the chrony to shake and turn on the tripod, meaning lower readings as the bullet has a bit further to travel between the sensors.

Thanks all. Yeah, I thought it was weird that I got so many repeats of exactly the same velocity. Will find somewhere I can position it further out as advised. That explanation for the lower-than-anticipated velocities is interesting, @Cigar. Will give the 41.4 & 41.6 another go before doing anything else. Powershoks were 100 grain, so I was hoping to get the 95 grain Targex going faster.

3130fps for the Powershok 100gr seems pretty fast, Federal quote 2960fps.