Chrono Questions for Ballistics Gurus

I've finally gotten around to start measuring my handload velocities with a CE Pro Chrono Pal.  Aside from Labradar I'm assuming all chronos' velocity measurements are basically the same in terms of their internal detection electronics.   Using 10-round groups of various loads (no crimp) I'm getting ranges of 35 to 70 fps within the groups which surprised me because I religiously monitor my powder drops and have only once, a year ago, had to adjust the volume.   When I set up the chrono I use cardboard sight aids I made attached to the diffuser legs to get the bullet path dead center in x and y within the legs from sight dot to target.  The center of the chrono is 10 ft. from the muzzle.  I think my setup is excellent as it pertains to measurement accuracy.

I've used the unit in bright sun with diffusers and when cloudy without the diffusers.  Both conditions basically yield the same results.  What's really baffling is Starline unfired brass has significantly more fps flyers than R-P range brass fired 4 times.   The averages are very close when firing from my S&W 986 and 5"RO9 and increase by about 25 fps when seating to 1.120 COL opposed to 1.150, which I would expect due to higher pressure at 1.120.  It's the seemingly large spread of data in each load that bothers me.  So far I've shot the same 4 load variations in each of the pistols, firing 5 rounds of each then repeating the sequence.  The best10 measurements gave me a range of 24 fps at 1.120 COL in my RO on a cloudy day but the other 3 groups measured a significantly (not statistical due to the small sample size) larger range than the 986.

Is there something I'm missing?  Also the Hodgdon data for CFE say I should be around 1060 with 5.3 gn and 1185 with a max. of 5.9 using 1.125 COL. in a 4" bbl.  I'm getting an average of 1170 with 5.4 gn. 1.120 COL 5" bbls. and have individual readings in excess of 1180.  Could there be that much increase with an extra inch of bbl. and .005 less COL?  I do use check weights and my 505 beam setting is always on the money.  Any corrections or suggestions form the ballistics people?  As usual my tinkering leads to more questions and no conclusions.

Dave

I would point out a few things. The first is that a low SD and ES doesn't necessarily translate to accuracy, and it's not necessarily important. Velocity matters in rifles because of the distance a rifle bullet is expected to travel. Large deviations in velocity mean large deviations in bullet drop, among other things. But at the ranges we're concerned with, there's really very little drop between 25 and 50 yards--under an inch in most cases, total. Even if you increase the amount of drop by 20%, it's almost unnoticeable in terms of groups.

The second thing is that the variations you're observing are quite small. An extreme spread of 20-40 is not a lot.

Reloading company data is also just that--their data. It was exactly true at only one point in time: the day they recorded it, with the equipment they recorded it on, the bullet they used, the lot of powder they tested, the barrel they shot it through, and the guy that did the work. Change any of those variables, and the data changes.

As to the barrels, there are slow barrels, and fast barrels. The generally-accepted wisdom is that a slow barrel will produce lower average velocities than a fast barrel the next size down--5" to 4", for instance.

In short, you're over-thinking it

I agree with Keyholed you are over thinking it for handguns.  Nothing new I did the same thing for my first two years and still like to see those lower numbers.  If you read the gun magazines and look at the data charts they have for different guns and ammo you will see that it isn't the load that has the best ES and SD that produced the best group.  Don

Good.  I was hoping it was me wishing for the unattainable....and not the result of faulty equipment or loads, although not so sure about my loads.  I guess what I was missing was a lower expectation of imperfection. Thanks for taking the time to respond.  A good lesson.

I, too, have found that Extreme Spread doesn't really match up with accuracy until you start approaching the higher two-digit values.  If you're interested, I have a very old speadsheet of some of my testing from long ago that has a pretty fair amount of detail for some .45 loads and .22 ammo of the day.  The spreadsheet can be found at:

Excel Version of .22 and .45 Ammo Testing

It has group sizes and the velocities with Excel and Chrony generated values.

To back up something else mentioned earlier, the difference in drop between 1150 fps and 1190 fps at 50 yards is (mathematically) about 0.185 (~3/16) inch.  For various reasons, the actual difference in practice is a little more, but not much.

Are you striving to get the lowest extreme spread and lowest standard deviation?

Or are you reloading for cost savings, accuracy, and reliability in your pistol?

Back when I was benchresting, I found that my loadings with the lowest es and sd were definitely not my most accurate loads.

I know it doesn't seam logical but my most accurate loadings had about a 85 to 100 fps velocity spread and the ones with 20-25 fps spread would not group in my rifle. 

Bottom line is don't worry about it. 

Cranky

+1 on Cranky's comments..

Chronos are great with Rifle to help you with Max loads. On our rather low pressure loads I  only Chrono my most accurate loads to give me a velocity range which I use when switching powders(not that it always works)