Erratic velocity with pressure ladder

[letrbuck]

During two different pressure Ladders the other day, I had some erratic velocities. Have not had this with this rifle and powder combo. Neck tension issue? Chronograph issue? Pressure issue?

280 AI and SB 6.5 powder, virgin peterson brass

168 Berger vld
55gr--2855fps
55.5--2923
56--2880
56.5--2932
57--2958
57.5--3011
58--2982


162 Hornady Amax
55--2937
55.5--2928
56--2942
56.5--2978
57--3011
57.5--2993
58--3066

 

[Jud96]

What primer? What action? Seems like inconsistent ignition. I’ve not used that powder, but maybe it needs a hotter primer to help ignition? Also, if your action doesn’t have sufficient ignition energy and/or firing pin drag it could cause inconsistencies.

 

[207North]

How many shots per powder charge?

 

[letrbuck]

How many shots per powder charge?

Just one each, but I have gotten linear relationships with this rifle and powder with other bullets I've tried, and good SD/ES. The up and down velocities with increasing powder charge just seems off to me

 

[207North]

Berger numbers



Seems reasonable to me considering whatever the ES might be for each powder charge. More shots would tell you the whole story.

 

[IAm_Not_Lost]

I mean, for that powder amount and general velocity you’re only moving up a few feet per second per 0.1 grain increase in powder. So, depending on your SD, an increase of 0.5 grains might basically be around what your SD is, leading to only one shot per load change being a bit misleading at times.

Not to mention just the general modern thought process that reloading ladders and nodes aren’t really as beneficial as we thought. The data from Hornady and Litz are pretty interesting.

 

[letrbuck]

Was not searching for a node, was trying to get an idea of where pressure was.

I just wasn't expecting velocities to increase and drop that much between 55 and 58 grains of powder

 

[HuntHarder]

Looks reasonable to me

 

[SDHNTR]

I usually see more erratic velocities in brand new brass.

 

[IAm_Not_Lost]

Was not searching for a node, was trying to get an idea of where pressure was.

I just wasn't expecting velocities to increase and drop that much between 55 and 58 grains of powder

Wasn’t saying that you were, was just throwing that out there.

But yeah, I think more shots per load would give you a better idea. I’ve noted new brass is more erratic as well, as some others have noted.

 

[hereinaz]

I usually see more erratic velocities in brand new brass.

This. I wonder what brass prep was done to uniform it and neck tension.

 

[Andouille]

The two factors I can think of that are known to have a substantial effect on SD are:
1. Deburring & chamfering the necks to ensure consistent bullet release. Discussed in the Hornady podcast and mentioned by either Eric Cortina or Brian Litz (my memory is fuzzy).
2. Powder measurement accuracy. Most powders give 5-8 fps per 0.1 grains, so being off 3-4 tenths could explain some of your error.
3. Low case fill and powder "sloshing" around in the case. You'll want to be >90% fill with SB 6.5 to avoid powder movement.

I concur that 2nd firing on brass typically gives a lower SD after full-length resizing, with all necks sized uniformly. Are you using cheap new brass that might have variability in sizing?

I typically get an SD around 12-15 with Staball 6.5 in 300 WSM, 30-06, and 6.5 CM with a good case fill. Not as tight an SD as H4350, but the higher velocities are often worth it.

 

[letrbuck]

The two factors I can think of that are known to have a substantial effect on SD are:
1. Deburring & chamfering the necks to ensure consistent bullet release. Discussed in the Hornady podcast and mentioned by either Eric Cortina or Brian Litz (my memory is fuzzy).
2. Powder measurement accuracy. Most powders give 5-8 fps per 0.1 grains, so being off 3-4 tenths could explain some of your error.
3. Low case fill and powder "sloshing" around in the case. You'll want to be >90% fill with SB 6.5 to avoid powder movement.

I concur that 2nd firing on brass typically gives a lower SD after full-length resizing, with all necks sized uniformly. Are you using cheap new brass that might have variability in sizing?

I typically get an SD around 12-15 with Staball 6.5 in 300 WSM, 30-06, and 6.5 CM with a good case fill. Not as tight an SD as H4350, but the higher velocities are often worth it.

Virgin peterson brass, but a new lot. The other lot I have gave me an SD of 6 with a different bullet and staball 6.5.

I guess I got a little hung up on the numbers, and picking a charge to shoot for groups, given the velocity swings between 55 and 58 grains.

 

[letrbuck]

What primer? What action? Seems like inconsistent ignition. I’ve not used that powder, but maybe it needs a hotter primer to help ignition? Also, if your action doesn’t have sufficient ignition energy and/or firing pin drag it could cause inconsistencies.

Weatherby MKV action, 215 primers

 

[Andouille]

Probably not the primary cause of your issue, but if you like to tinker, you might be able to knock off a few fps of variability using G210M or BR2 match primers. No need for magnum promers as you're just above my SB 6.5 30-06 load (56ish gr) and still well below 300 WSM (68gr) where the match primer has worked well for me and many others.

 

[Jud96]

Weatherby MKV action, 215 primers

Well then it’s not the primer being too weak. If other powders aren’t as erratic, then your rifle is probably okay. Might just be that combination isn’t happy. You could try other primers as others suggested.

 

[Vern400]

Use two powder scales.
Use two chronographs if possible.
Could also be bad ignition - poor powder choice, primer compatibility, excess space in powder column.

Two days ago I was testing powder charges. Accidentally fired 45.5 gr on a 46 grain group. Chronograph picked up 40 ft per second different.
So I fired 10 shots, five at each powder charge.
Of course I used the same point of aim.
I separated out the data on the chronograph, and I got two tight groups separated by 3/4 in paper.
Totally the opposite of what's happening with you.

If you're actually overlapping velocities, your extreme spread and SD are really poor.

 

[HuntHarder]

Here are the last 2 pressure ladders I did with H1000 and H4831sc.

 

[HuntHarder]