velocity goes down with more powder

[Frankr]

I have seen this twice now in my lifetime wondering if anyone else has seen it or can explain what is happening.
I was working on a load testing in different temperatures. as I always do. I have a load that for example: I shoot 40 grs of powder and chrony 3,039FPS then I shoot 40.5grs of the same and chrony 3,060 FPS then I shoot 41.0grs of the same and chrony 2,990 to 3,020. now these are kind of hot loads. but no heavy bolt lift or brass flow in the ejector, plunger marks on head of case, primer pockets tight. slight primmer cratering I fell is more a charistic of bolt face design and fireing pin and soft primer cups.

 

[JamesnTN]

Yes I have mainly in wildcat cases, as it goes down it'll spike up again and depending on the cartridge it can spike as much as 150 fps within just a few tenths of a grain.
Loads rise and then they hit a plateau, then you'll se them fall off and the spike and begin rising again until they plateau once more. Be careful and read your cases and look for extractor marks.

 

[chkunz]

This is interesting and it brings up a question I have had in my limited experience with using a chronograph. What is the accuracy we can expect from these instruments? For example if I could fire ten shots that were exactly the same velocity what would the variability be in the readings from the instrument? The answer to this question could shed some light on your question if the variability is large enough.

 

[BillPa]

I've only witnessed it once. I was working up a load for a new 30-338WM I put together shooting 180 NSBs increasing the charge weight .5 grain at a time using IMR4350. At some point the velocity started dropping off so I reduced the charge and back up it went. I figured it was a fluke so I tried it again with the same results three times. Everything was normal, bolt lift, primers etc. BTW, this was during the same range session within an hour or so.

Bill

 

[BCoates]

I saw this with my 6.8 SPC and JamesnTN is right, if you keep loading progressively higher charges the velocity can and will spike up. I assume the amount of the spike varies dependent on case size.

 

[SeabeeKen]

I saw it when doing load development for my 243 AI. H-414 gave very erratic pressures. I found that lower charge weights gave huge pressure spikes, flattening primers even though I was within charges that were fine in a 243 Win. As charge weights went up, things got better and eventually pressures went back down to normal. I haven't used it since.

 

[mikecr]

I've never seen it, and it isn't mentioned often. Can't be normal.
What would cause it that is repeatable?

 

[Preacher]

I noticed years ago, that while working up a load with progressively higher charges, that when you got to a certain point the velocity would not go up as much as the previous weight did. It always seemed to be right at the point of to much powder, and approaching max weights for that cartridge...
Some folks say it don't happen, but it did to many times to not pay attention to it...

 

[CatShooter]

This is interesting and it brings up a question I have had in my limited experience with using a chronograph. What is the accuracy we can expect from these instruments? For example if I could fire ten shots that were exactly the same velocity what would the variability be in the readings from the instrument? The answer to this question could shed some light on your question if the variability is large enough.

We (shooters) tend to question a lot of things, but we seem to have a blind faith in electronic devices. If your digital scale said 47.3 grains, we believe it, despite the fact that people have been talking about their drift and inaccuracies for years.

I have owned chronos from the beginning of time - my first was a kit that used a R/C circuit and 35mm film with silver paint, and the bullet broke the circuits, and you used a chart to converted a volt meter reading for the velocity... it was "interesting".

My second was a Hollywood that worked on the same principal (and was no more accurate, but had enormous "hi-tech" appeal) - I still have it in the museum.

I went to the Oehler 33 when they first came out, then a 35-P and two CEDS. I have them all.

The one thing I have learned is that if you have blind faith in chronos, I have a bridge in Arizona to sell you.

Line up 4 or 5 chronos in a row, and shoot through them all... you are in for a big surprise.

 

[chkunz]

This is interesting and it brings up a question I have had in my limited experience with using a chronograph. What is the accuracy we can expect from these instruments? For example if I could fire ten shots that were exactly the same velocity what would the variability be in the readings from the instrument? The answer to this question could shed some light on your question if the variability is large enough.

We (shooters) tend to question a lot of things, but we seem to have a blind faith in electronic devices. If your digital scale said 47.3 grains, we believe it, despite the fact that people have been talking about their drift and inaccuracies for years.

I have owned chronos from the beginning of time - my first was a kit that used a R/C circuit and 35mm film with silver paint, and the bullet broke the circuits, and you used a chart to converted a volt meter reading for the velocity... it was "interesting".

My second was a Hollywood that worked on the same principal (and was no more accurate, but had enormous "hi-tech" appeal) - I still have it in the museum.

I went to the Oehler 33 when they first came out, then a 35-P and two CEDS. I have them all.

The one thing I have learned is that if you have blind faith in chronos, I have a bridge in Arizona to sell you.

Line up 4 or 5 chronos in a row, and shoot through them all... you are in for a big surprise.

That was right on point to my question. So maybe what the OP is seeing is not a decrease in velocity with an increase in powder which is very unlikely but the variability in the readout of the instrument, very likely. Not many folks have an appreciation for what to expect from electronic instruments plus there is a lot more going on in addition to the electronics when using a chronograph. I have limited experience with using a chronograph and was not impressed with what I have seen with it so far. This would be a good topic for a new thread to see if we could get the benefit of some folks who have extensive experience with using a chronograph and also have experience and an understanding of the uncertainty of electronic instrumentation.

 

[BoydAllen]

So, if you get an unexpected result, blame the measuring tool? What we have here are multiple examples, from several shooters, some of which retested and had the results repeat. It seems more likely to me that what is going on is an oversimplified and incomplete understanding of what is possible.

 

[mikecr]

So what would cause it?

 

[BoydAllen]

I don't know. This may be one of those things that we are aware of, learn how to avoid, but do not fully understand how it works. So often, I see guesses defended as if they were facts.

 

[chkunz]

So, if you get an unexpected result, blame the measuring tool? What we have here are multiple examples, from several shooters, some of which retested and had the results repeat. It seems more likely to me that what is going on is an oversimplified and incomplete understanding of what is possible.

My experience is that when you see the instrumentation disagree with the laws of physics the instrumentation is suspect and needs to be examined closely. As I said I am not experienced with chronographs and I invited those that are to offer their knowledge and experience and we have one reply that has done that. Also I have never heard of a case where an increase in powder charge results in a decrease in velocity, maybe someone can offer and explanation consistent with the physics. Some of the responses that you cite on this thread talk about spikes in pressure and not the reduction of velocity as a result of the increase in powder charge. In either case we do not have a lot of details about any of these reports to form a sound conclusion. The report on the reliability of chronographs was helpful.

 

[BoydAllen]

We do not know that the phenomenon described violates the laws of physics. All that we know is that was unexpected. I am aware of the limitations of chronographs, but when experiment is repeated and the same thing happens, I would tend to discount random instrument error.

 

[chkunz]

We do not know that the phenomenon described violates the laws of physics. All that we know is that was unexpected. I am aware of the limitations of chronographs, but when experiment is repeated and the same thing happens, I would tend to discount random instrument error.

Has anyone offered a theory of what would cause this? If we do not know the cause it is possible that a plausible theory might lead someone that has the capability to investigate. Or it simply may not be worthwhile at this point.

 

[Frankr]

well guys first of all you can never duplicate a load in a different rifle,there are too many variables. bore and groove diameter,twist variation,throat and neck clearance,powder lot burn rate difference,amount of moisture in same lot's of powder,temperature,air denisity. barrel length's. ect. BUT I have shot the same load in a completely different rifle and got the same results in group size and velocity.

I'm going to just tell you about it. I was shooting a 300WSM a few years ago,sporter barrel 24". I was trying to push a 180 NBT to 3,100 FPS and I could not. so I went to a slow powder, I think it was RL22 or RL(sorry I cant remember witch and how much). but it was a full case. and as I went up on the powder, the chrony had been working fine. I do not remember the exact speed but when I went up to the next load 1/2 gr more powder the velocity was slower, I was shocked..so I went another 1/2 gr more and the velocity was slower and the bolt was very hard to open, I quit. went to a faster powder lighter bullet and I was happy.

I have been shooting through a chrony since 1996. I have seen a lot a weird velocity reading, errors. no reads. to just not working. But I have never seen more powder go slower. so I kind of just forgot about it. but I have a few friends who say they have see it and even a few forum members.

Now I have been shooting a 6xc in a hunting rifle for around 2 months now. I have shot a lot of stuff threw it. it has shoot well. it's a 26" sporter barrel. well it has been suggested to me that H4350 is the go to powder. well my rifle like 40.0grs of it but the velocity has been low compared to what I can get with other powders but it has shot great and has been very consistent even at long range. I have chroney'ed this load a dozen times and it is consistent at 3,039-3,040 SD ES around 20FPS at worst I have seen. But I felt it would shoot better up 1/2 gr of powder, I even thought I might see 3,100FPS. well it did shoot better but the velocity gain was @20FPS. well I went up to 41.0 grs and the load went slower at 2,990 I was like ah I have seen this before. not good. the group was poor two cutting one a inch away. just too hot.
Now i'm not,or can not rule out chrony error. but I have never seen more powder go slower as a chrony failure.I have see many times. no read, 3,500 FPS, error. I have seen a lot of weird stuff but never consistent slower velocity's with more powder.so i'm not sure what went on there and not quite sure I want to keep testing it to find out. I have been loading since I was 15 years old and have never blown a primer or any thing like that. I feel I have allways had a good feel were to quit and where I could push. I this just feels weird.

that is one accurate shooting rifle. but all my hunting rifles shoot like this, if only my bench rifles would.lol...
Ah there is a error on that target. it's 4350 not 5350. ah that 5350 powder caused this whole mess.
Hey I have even thought wonder if I messed up. But i'm quite sure I did not.

 

[Frankr]

We do not know that the phenomenon described violates the laws of physics. All that we know is that was unexpected. I am aware of the limitations of chronographs, but when experiment is repeated and the same thing happens, I would tend to discount random instrument error.

Has anyone offered a theory of what would cause this? If we do not know the cause it is possible that a plausible theory might lead someone that has the capability to investigate. Or it simply may not be worthwhile at this point.

My theory is. you guy's are going to laugh and make fun of me. it is a slow powder for the case, smokeless powder's do not need oxygen to burn. but there is just enough oxygen inside the case to help the burn or speed up the burn. now more powder filled the case so there was no oxygen which slowed the burn or burn speed down.
hence the slower velocity. or may have just slowed the pressure curve down to where the bullet exited the barrel as pressure peeked, so as you had high chamber pressure but that pressure never had a chance to push the bullet. now I feel you could change anything that could change the pressure, as the list is long. and not see it happen. even internal cases volume. like a different lot of cases. as I said the list is long.

 

[chkunz]

We do not know that the phenomenon described violates the laws of physics. All that we know is that was unexpected. I am aware of the limitations of chronographs, but when experiment is repeated and the same thing happens, I would tend to discount random instrument error.

Has anyone offered a theory of what would cause this? If we do not know the cause it is possible that a plausible theory might lead someone that has the capability to investigate. Or it simply may not be worthwhile at this point.

My theory is. you guy's are going to laugh and make fun of me. it is a slow powder for the case, smokeless powder's do not need oxygen to burn. but there is just enough oxygen inside the case to help the burn or speed up the burn. now more powder filled the case so there was no oxygen which slowed the burn or burn speed down.
hence the slower velocity. or may have just slowed the pressure curve down to where the bullet exited the barrel as pressure peeked, so as you had high chamber pressure but that pressure never had a chance to push the bullet. now I feel you could change anything that could change the pressure, as the list is long. and not see it happen. even internal cases volume. like a different lot of cases. as I said the list is long.

I asked for a theory and you have given us a theory I am not going to laugh or make fun. Now I am not a forum bully but let me counter your theory. When I was in the business of testing upper stage solid propellant rocket motors we tested them at simulated altitude, near zero pressure, (think no oxygen), etc. to simulate how they would perform in flight. We compared their performance to what it would be at ambient conditions 14.2 to 14.7 psi with the oxygen level we breath and the ignition performance was very similar with no oxygen and the normal twenty percent level we breath. There is a lot more technical detail that I could add but my opinion based on that experience is that the additional small amount of ambient oxygen is not a factor. The propellant contains both fuel and oxidizer and a small amount less of oxygen in the case is not significant. Some of these solid propellant rocket motors I am referring to were single base propellants and were were similar (the space shuttle booster propellant is identical to IMR powders) to many of the powders we use for loading rifle cartridges. You gave us a theory and I appreciate it very much, it keeps the conversation going, now someone else go get us another theory. Take care, Clyde.

 

[Frankr]

I will counter your theory with in your testing you were not pressurizing a confined space. or tube. so the pressure curve can change drastically.. by just changing bullet seating depth .010. . I am going to retest through a different chrony were it's at and .010 into the rifleing. I'm set up at just touching right now but i'm betting if I was .010 more in I would have seen more velocity and high pressure.
But hey just a theory. I really do not believe it to be a chrony error.

will be the weekend of 10/04 that I will have time to do it.

T-REX thanks for your input. as to your question how accurate is a chronograph. the manufacture's claim 99.5% accuracy .
now IMO they are only as accurate as the guy who sets them up. if you want true reading's you need to measure out to it. shoot straight over the sensors and they seem to work quite well. now if you don't set it up correctly or shoot threw it on a angle you will get bad. sometimes light can throw them off or out of wack but you will usually notice it's not right. I have set up many of range cards for a lot of rifles that were spot on from the velocity readings I have got and also tuned loads and have seen the benefits at long range from that so I feel they work great and accurately most of the time.