Help me understand this-Velocity is going down??

[DocGlenn]

Reloading 6.5Creedmoor. Ruger American, nothing fancy. Using IMR4350, brass is factory Hornady, trimmed. Working up a load with new powder. 41.5gr got me 2623 fps, SD 6.09, but when I went up to 41.7, the speed DROPPED to 2609 fps, SD 13.01??? I'm using a Labradar. I can hear the powder compressing when seating the bullet (loading to mag length). Could the extra compression of the heavier load be casing the reduced velocity?? The 41.5 load is great, don't want or need to go any faster, but I don't understand the physics of what is happening here? Any insight would be much appreciated. Thanks!

 

[338 dude]

Reloading 6.5Creedmoor. Ruger American, nothing fancy. Using IMR4350, brass is factory Hornady, trimmed. Working up a load with new powder. 41.5gr got me 2623 fps, SD 6.09, but when I went up to 41.7, the speed DROPPED to 2609 fps, SD 13.01??? I'm using a Labradar. I can hear the powder compressing when seating the bullet (loading to mag length). Could the extra compression of the heavier load be casing the reduced velocity?? The 41.5 load is great, don't want or need to go any faster, but I don't understand the physics of what is happening here? Any insight would be much appreciated. Thanks!

Basically I believe it could be too much powder not all being completely burned before it exits the barrel

 

[BoomFlop]

My best guess is that with the compressed load, you are getting CBTO/COAL variance of the powder pushing the bullet out of the case causing slightly more capacity. This is reducing the pressure, causing a reduction in velocity even with the increase in powder.

This theory would be fairly easy to check. Load a round and check it the next day or maybe even immediately.

Could also be neck tension variance, especially if you are not testing velocity over 5-10 shots (as you didn't specify whether it repeated).

Good luck
Steve

 

[jimbires]

were both loads shot on the same trip to the range ? this difference in velocity could be caused by outside temp .

 

[DocGlenn]

were both loads shot on the same trip to the range ? this difference in velocity could be caused by outside temp .

Yes. Same day, same time. Each string was 6 shots.

 

[QuietTexan]

Short answer - yes, velocity can and will decrease at certain points with an increased charge weight.

I don't understand the physics of what is happening here? Any insight would be much appreciated. Thanks!

You asked for a long answer Probably a confluence of too many things to ever nail down a solid "why". Change in the timing of the gas going around the bullet before engraving, changes in internal ballistics due to being at a different point for pressure/burn rate, change in how the primer ignites the column, bullet seating change from being pushed out, inconsistent powder packing if you aren't using a drop tube. So many things it could be.

Not sure how many shots you took at each charge weight, My man! 6 shots at each weight is commendable, and actually gets to the edge of being inferential. No less than 5 shots for numbers is my general opinion, but as many as possible is best. There's a mathematical answer here that could be anything from "yes it's actually slower" to "it's not really slower, it's just a very unstable load and/or you don't have enough data points to show that it's really faster but you're so likely to get a slow load that you can't really use the load".

Typical results of a ladder test - are the decreases real? Or are they statistical lies where a 95% percentile round was shot right before a 5% percentile round?

Source:

Creighton Audette’s Ladder Test

Creighton Audette came up with a method for developing precision rifle handloads that has been refer...

precisionrifleblog.com

In practical terms we avoid the messy areas and focus on the more promising flat areas because it's more likely they actually are stable than it is that the unstable areas are actually stable. Not a lot of hard evidence on what makes bad things bad because not a lot of people are lining up to make bad loads on purpose. I do it sometimes - if I find a bad load I'll keep it written down to use to test something specific or see if it can get one part of it tuned better, but generally I work from good to better. Mikecr makes a good point in that testing the effects of some changes needs to be done intentionally outside of a node so that the changes aren't being obscured by another factor compensating for those changes. Maybe we should do more "bad" work to learn from it.

Edit: You posted while I was typing, had to change my assumptions.

 

[HSmithTX]

This ^^^^^^^.

 

[MagnumManiac]

Compressing a powder changes the burn to a SLOWER event….this can cause pressure to drop, hence the lower velocity.
I do not normally see this in cases as small as the Creedless, but believe it can happen.
Air space in the case can cause a FASTER event, so the opposite is very true.
This is often why when adding more powder the velocity steadily drops until it bottoms out and stays there….this is bad juju and should be avoided at all costs as a pressure excursion can happen at ANY time doing this. The 243 is notorious for this phenomenon.

Cheers.

 

[Doom2]

Statistically the difference is significant. Likely the compression is the cause but we really don't know for sure. Compressed charges can and in some cases do change CBTO over time. I recently went through this with N135 in 223.

 

[7070yshot]

Reloading 6.5Creedmoor. Ruger American, nothing fancy. Using IMR4350, brass is factory Hornady, trimmed. Working up a load with new powder. 41.5gr got me 2623 fps, SD 6.09, but when I went up to 41.7, the speed DROPPED to 2609 fps, SD 13.01??? I'm using a Labradar. I can hear the powder compressing when seating the bullet (loading to mag length). Could the extra compression of the heavier load be casing the reduced velocity?? The 41.5 load is great, don't want or need to go any faster, but I don't understand the physics of what is happening here? Any insight would be much appreciated. Thanks!

1) not all powder is burning
2) variation in neck tension
2a) case weights are lighter than other string. A light case has more internal volume, this the energy has more room to expand, heavier cases the energy has less volume to consume during expansion so it is used in pushing the bullet
3) was rifle barrel clean for each string of tests?
4) environmental factors?
5) sometimes it just does that. The physics are E=mc2, and I'm not being a smart ***. The energy goes somewhere, it's either consumed by expanding the brass for bullet release, build up of pressure, bearing surface drag, or wastes by continuing to burn outside if barrel.

 

[Lenny Foffa]

Just a question here, but would a magnum rifle primer help at all with this velocity loss with the compressed load????

 

[skipglo]

Basically I believe it could be too much powder not all being completely burned before it exits the barrel

Yup!

 

[skipglo]

Reloading 6.5Creedmoor. Ruger American, nothing fancy. Using IMR4350, brass is factory Hornady, trimmed. Working up a load with new powder. 41.5gr got me 2623 fps, SD 6.09, but when I went up to 41.7, the speed DROPPED to 2609 fps, SD 13.01??? I'm using a Labradar. I can hear the powder compressing when seating the bullet (loading to mag length). Could the extra compression of the heavier load be casing the reduced velocity?? The 41.5 load is great, don't want or need to go any faster, but I don't understand the physics of what is happening here? Any insight would be much appreciated. Thanks!

Stay at 41.5 and leave the science out. You will P.O. the reloading God's!

 

[brant89]

The Ruger American rifles have pretty short barrels, so my guess is that you've hit the max amount of 4350 that will get a full burn, and the compression is sucking a little energy out of it.