.300WM Chrony results: Should I be disappointed?

[wombat13]

I used my chronograph for the first time yesterday to check the velocity I'm getting with my handloads. The variation in velocity was more than I expected. Is a standard deviation of 21 fps typical? Should I be disappointed? If so, what should I do to try to reduce the variation? Velocity results and component info is below:

10 shots over ProChrono Digital
Temp 30F
Chrono was 15ft from muzzle; sun diffusers in place

Max vel: 3,072
Min vel: 3,003
ES: 69
Average: 3,055
St. Dev: 20.9

Note: The cold bore shot was 3,003 fps. The next lowest was 3,023.

Load is 165 gr. Barnes TSX over 72.8gr IMR 4831 with Rem LRP and Win Brass (twice fired, trimmed if necessary). COAL is set to the max that will feed in the magazine (rifle is Ruger Hawkeye).

I use a Dillon RL550 to load, but I don't use it as a progressive. I size/deprime and then measure for length. I then prime, rotate, drop a charge, remove the case and measure the charge on my scale, trickle in powder until I reach my weight. Put it back in the shell holder, rotate into position and seat the bullet. I know my powder variation is not more than 0.05 grains according to my scale.

Thanks for any comments. I've never used a chrony before, so I don't know what "good" velocity results are.

 

[rsrocket1]

If you look at "barrel harmonics", there is a good theory which shows that longitudinal waves can cause the mouth of the case to be loose at one time and tight at a different time. This affects the peak pressure and subsequently the MV. More importantly, the waves determine whether the muzzle is loose or tight when the bullet exits the gun and when it is tight, you get the best consistency in groups.

Your SD's are 0.6% which is pretty good. The groups are what's important.

While you may not be able to perfectly control the instant the bullet exits the case mouth, you do have more latitude when it comes to the bullet leaving the muzzle. That will result in a good small target group and that is what you should be striving for. I've found very good groups even with 1-2% differences in SD MV's. When I am below 1% SD, I figure the load is pretty darn consistent and if the accuracy isn't there, try a different charge weight.

Here's some good reading on optimal barrel time. It seems to work fairly good for me with .308 and .223 in picking loads for good accuracy. Quickload is only provides an estimate but it saves a lot of time in getting close to a good starting point where you're not loading across many grains of charge looking for a good group.

 

[Blue68f100]

It does not take much to impact crony readings. Just a change in cheek pressure, shoulder position, or any thing that impacts the gun movement. So your number are not that bad. Before I got my crony I had worked up loads decades ago without one. I did crony some of them that always shot 1 moa and the numbers were all over the place. What counts is the target, the crony numbers are nice for drop calculations. I use to always do it old school. Shot target at different range to get the drop.

 

[wrench459]

The first rounds that I put across a chrony was with factory ammo.
115gr Federals 3.1 bbl.
1027
1018
1033
1007
ES 26 SD 11

Then with a 16.5 inch barrel 115gr WWB.
1344
1357
1313
1384
1322
ES 71 SD 28

A chronograph is just a tool.

Now for a reload 124gr EGA JHPV2
1409
1359
1400
1418
ES 59 SD 26

 

[wombat13]

Thanks for the info. The load has always been accurate for me and now I know the chrony results are good as well. I got the chrony because I have a Leupold VX-6 and I can get turret caps indexed to my load, but I need to give them the bullet’s MV.

I might try using new cases and see what happens to the SD.

 

[Blue68f100]

The key is too allow the gun to move the same each and every time. I use a Lead Sled when working up loads to help hold the gun more solid than me. A 300 WM is a high recoil gun and can make this harder to do.

Just remember it what counts on paper. If your going to provide data for turrets include all weather conditions and load data. Best to confirm on the target.