Let's talk about chrono stats for a moment

[STFU]

When I start working on a new load, I usually run 10 rounds of each powder level I want to try.
(EG: 10rounds @ 4.2 gr, 10rounds @ 4.25 gr, and 10rounds @ 4.3 gr.)

When I collect chrono logs and look at the stats, I always seem to have one round (usually the first) that skews the whole sample.
I am now at the point where I want to shoot 10, throw out the high, throw out the low and calculate the stats off of the remaining eight.

Does anyone else do this?
Is my thinking on this really flawed or is this normal?

 

[Aszerigan]

I always throw out the top and bottom velocities. Gives a more accurate view of actual velocities. One bullet isn't sized properly, it slows down. One is undersized, it speeds up. Nothing is perfect in manufacturing, so tossing the anomalies makes sense to me.

Assuming everything is equal, then the velocities should be very similar - lower SD - and tossing top and bottom shouldn't affect the average anyway.

We noticed that your first shot even on a hot bore was noticeably slower than the rest. I can not explain that.

 

[gabrigger]

I always truncate the high and low velocities. This leads to more consistent data and takes out the "quirks" of the chronograph. My first shot seems to always be an outlier in the data for some unknown reason.

 

[Aszerigan]

When I start working on a new load, I usually run 10 rounds of each powder level I want to try.
(EG: 10rounds @ 4.2 gr, 10rounds @ 4.25 gr, and 10rounds @ 4.3 gr.)

Also, in my opinion (so please ignore if you wish), I would work up in larger increments. One half of one tenth of a grain isn’t going to produce significant changes in velocities and would be nearly impossible to replicate on a progressive press without some very precise - (and very expensive) powder measures.

I’d probably go 4.2, 4.4, 4.6 and check accuracy on those for a more precise zeroing of an accurate production round.

 

[NyleRN]

I always throw out the top and bottom velocities. Gives a more accurate view of actual velocities. One bullet isn't sized properly, it slows down. One is undersized, it speeds up. Nothing is perfect in manufacturing, so tossing the anomalies makes sense to me.

Assuming everything is equal, then the velocities should be very similar - lower SD - and tossing top and bottom shouldn't affect the average anyway.

We noticed that your first shot even on a hot bore was noticeably slower than the rest. I can not explain that.

Is this something you do with match rifle projectiles or just pistol? I ask because I'm wondering how consistent match projectiles such as Berger or Sierra are. I've never sampled a lot from a batch I've bought with my calipers

 

[Gaffer]

I ran into the same issue. The first shot was always the one off. Might be a cold gun or cold crono?? Anyway, I always shoot a few rounds through the chrono with the gun I am using (using rounds I am not testing), then start my testing on my reloads. The reloads seem much more consistent when I use the above procedure.

Ron

 

[STFU]

Thank you all. Very helpful info.

 

[STFU]

We noticed that your first shot even on a hot bore was noticeably slower than the rest. I can not explain that.

Indeed we did. And I had the same thing happen the other day with the new Xero C1. (Which I adore...thx for the recommendation.)

I have even gone so far as to pull 5 random loads from a sample of 100 and check the powder contents. All were within .25 grains of what I wanted. Not sure why I always get those fliers, even when the barrel is warmed up, etc. Weird.

 

[Aszerigan]

Is this something you do with match rifle projectiles or just pistol? I ask because I'm wondering how consistent match projectiles such as Berger or Sierra are. I've never sampled a lot from a batch I've bought with my calipers

Across the board, rifle and pistol. Most every bullet is consistent but every once in a while you get an out of spec projectile that messes with your averages.

 

[MrMunster]

Consider running a few shots of a known load through the gun and over the chrono first. It should help solve the cold gun/cold chrono & provide you a comparison to previous data.

 

[DadSmith]

Across the board, rifle and pistol. Most every bullet is consistent but every once in a while you get an out of spec projectile that messes with your averages.

So shoot 12 instead of 10 and take off the fastest, and slowest velocity?

 

[Trapper Jim]

I shoot to foul the barrel and set the Chrono as for whatever reason the first reading is usually low. Then there is the fact that I’m not sure Weights and Measures certifies Chronos no matter what we pay for them?

 

[Hawkeye7br]

I'm not seeing any real numbers being used? A rifle averaging 2700 fps with an extreme spread of 27 has a +/- variance from average of only .5%, correct?

Then there is the fact that I’m not sure Weights and Measures certifies Chronos no matter what we pay for them?

Agree. Does changing light (cloud, humidity, wind refraction of lighting) have micro effects on chronograph readings?