Is my crony messed up?

[RM100GUY]

I was shooting some long shot strings with my Wildcat Mk1 .22 and Streamline .22 after polishing the bore's, I used #7 polishing compound from Ace Hardware for car paint, results appear to be excellent so far. Both guns had been shot some

since polishing I'm looking for more shots between cleaning in both guns, shot the Wildcat first across my ProChrono, 62shots, high 880, low867, Average 875, Extreme Spread 13, Standard Deviation 2. Next the Streamline, 63shots, High 869, low 857, Average 863, Extreme Spread 12, Standard Deviation 3. Question, how can the Streamline have a higher Standard Deviation with a lower Extreme Spread? I've had this crony for at least ten years and I always believed it's readings. By the way after several hundred pellets my last 5 shot group with the WC was still pellet on pellet, haven't shot SL, enough to tell improvement yet. Pictured group was last shot in WC 25y.

 

[STO]

Your extreme spread is the absolute difference between your highest and lowest velocities. You standard deviation is.... a little more complicated, but reflects how closely your velocities cluster on average. Here is how SD is calculated:
https://www.mathsisfun.com/data/standard-deviation-formulas.html

Putting it in simpler terms, say you shot a ten shot group on paper. Your extreme spread would be the greatest distance between impacts. Your standard deviation would be a reflection of the majority of the group's cluster. So if you had two ten shot groups and one had a huge flier but otherwise was very tight, it'd have a higher extreme spread but lower standard deviation. 

I hope that helps/made sense. 

 

[mmahoney]

Both shot strings are great. The extreme spreads are only one fps apart (practically identical). The standard deviation is telling you how close to the average velocity a typical shot is likely to be. The data is saying that the Wildcat is slightly more consistent at shooting at steady velocity, but that it had a shot or two that opened up the extreme spread. The Streamline was slightly more erratic but stayed in a slightly tighter velocity range overall.

Normally the extreme spread and standard deviation track pretty closely but they are not the same thing and can deviate like in your example. Nothing wrong with your chronograph.