Velocity Spreads

[NCswamp]

I am trying to isolate the reason I am seeing high velocity spreads with my 28 Nosler loads. I recently shot a OAL length ladder - 15 rounds in groups of three. My velocities were all over the place in some case as much as 100fps!! I even shot a group of factory ammo and it had a spread of almost 30fps. In several instances I had single digit spreads for two rounds then a round 100 fps higher or lower. i also had four errors from the chronograph which seems high.


I believe that one of the following three things occurred: human error, variance in new brass, or bad chrono measurement

I acknowledge that sloppy work or human error is possible but I've never seen numbers like this. I individually trim each brass to .002, weigh each charge to 0.1 gr, and measure each bullet and brass / cartridge. I have not routinely weighed brass or bullets. I was using new Nosler brass (never been fired), 168 gr ABLR bullets with US 869 powder, and no crimping. We had wind blowing 4 to 8 at 6:00 and I did notice the chronograph bouncing slightly.

I would appreciate some help...thanks

 

[mjcmichigan]

30 FPS for factory ammo is not starling.

Single digits is great!

I look at Neck tension when things are inconsistent.

You said it was new brass, I’ve seen a lot of variability in new brass. I will resize it.

Did you trust it was sized properly?

Also, some power primer combos don’t behave as well as others. You can go to bench rest primers. Or try another brand.

It should be solveable.


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[Nimrod84]

Welcome.

What mjmichigan wrote is dead on. A lot of virgin brass isn't the best to do serious load development with - most brands aren't terribly consistent with their neck sizing and thus fliers show up due to different neck tension. Neck tension also can affect velocity - a lot of tension can result in more velocity while less tension can result in lower velocity.

Different primers can help a lot - too strong or too weak can cause poor ignition which results in large fps variance.

I would add that with my 1' folding optical chronograph, I will see up to 100 fps variations depending on light conditions. It will show single digit fps variance and their is a change in light conditions (cloud, bird, leaves blowing, sunshades bouncing) and it will go + or - 50-100 fps. I normally only see 20-40 fps light induced variance with an Oehler 35 chronograph with 4' spacing.

 

[Dr. Vette]

To follow on NM84, what chronograph are you using? If it's an optical then that may be the issue. If a Magnetospeed or LabRadar, then I'm going to put a lot more stock in the results.

I have one of each (Chrony, MS, LR) and I now use the LabRadar almost exclusively.

 

[joelkdouglas]

Took me a couple days to find it (the original post was corrupted or something).

http://www.mssblog.com/2019/08/22/reloa ... nsistency/




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[NCswamp]

Thank you all for your help. I listened to your input and tried a few things. I was able to do some re-testing yesterday.

1) I cleaned my rifle to remove the risk of copper fouling causing issues (friend's idea)

2) I resized the brass and made sure they were trimmed to within a .001 (mjcmichigan's suggestion)

3) Seated the bullet to +0.000 / - .0005 OAL (another friend's idea)

4) I placed the chronograph (Caldwell Precision) under a tent on a cloudy day to reduce light condition issues (Dr Vette's idea)

The result was rather fantastic. The velocity spread on 6 shots was 21fps (down from 100). If you remove the two fouling rounds it was 14fps. Even better, I had a MOA of .16 and .20 with a couple 2 shot groups at 200 yards. I This is what really matters to me is accuracy.

I will consider a new Chrono based upon the suggestions and also buy a deburring tool for the inside the case side of the primer hole (joelkdouglas's post).

Thanks

 

[Gun nut]

Congratulations NCswamp. Excellent suggestions here. I share the frustrations of the velocity spread. Definitely didn't have this issue when I started hand loading 55 years ago. LOL
Duane