300 PRC Velocity decreases every 100 rounds

[Doom]

@Mahziar,

First question is are these sample all with the same COAL/CBTO? It's not clear to me.

Also let me say you are trying to compare 5 shot samples shot on different days and lthat may have been loaded on different day. You cannot compare these small samples and make an informed judgement on whether or not these velocities represent differences that are statistically significant. It requires some further analysis which would require knowledge of the standard deviation of the two earlier samples. If you can post the actual data for all this I would be happy to explain this further.

 

[LCazador]

I don’t have the new measurement
But I know the fired is 0.5325-0.533 and the sized is 0.530

That is too much expansion of the head, in my opinion you are way over maximum pressure. Are you sure your measurements are correct? You should not have more than .001" expansion.

 

[Mahziar]

That is too much expansion of the head, in my opinion you are way over maximum pressure. Are you sure your measurements are correct? You should not have more than .001" expansion.

My chamber is not saami and it’s bigger at the 0.200, it measures at 0.535 on the reamer print. That’s why it expands more.

 

[LCazador]

My chamber is not saami and it’s bigger at the 0.200, it measures at 0.535 on the reamer print. That’s why it expands more.

So your sizing die does not match your chamber if it's sizing down to .530"? .005" is a lot of sizing of that area. What brand die? This may be part of the equation but things like an enlarged bore or advancing throat erosion make more sense.

 

[Mahziar]

So your sizing die does not match your chamber if it's sizing down to .530"? .005" is a lot of sizing of that area. What brand die? This may be part of the equation but things like an enlarged bore or advancing throat erosion make more sense.

Short Action custom modular sizing die
Before that I used Forster die for my first 2 tests, it was only sizing down to 0.532 which is only 0.001 but wasn’t sizing the neck all the way to the bottom like what short action custom does. You are right about throat erosion, the gun still able to shoot high 3s and low 4s and as of today I can not find any Bartlein HV in stock

 

[Mahziar]

@Mahziar,

First question is are these sample all with the same COAL/CBTO? It's not clear to me.

Also let me say you are trying to compare 5 shot samples shot on different days and lthat may have been loaded on different day. You cannot compare these small samples and make an informed judgement on whether or not these velocities represent differences that are statistically significant. It requires some further analysis which would require knowledge of the standard deviation of the two earlier samples. If you can post the actual data for all this I would be happy to explain this further.

It wasn’t the same cbto, due to throat erosion I had to increase the length
From first to the third test it was about 0.030 difference in cbto
I will send you the excel sheet
Thank you

 

[LCazador]

One more dimension to consider is how far that throat has advanced. Find dimension to lands and see how your CBTO relates. Obviously your difference of .030" in your CBTO that you stated is addressing the advancing throat erosion. How much jump?

 

[LCazador]

My chamber is not saami and it’s bigger at the 0.200, it measures at 0.535 on the reamer print. That’s why it expands more.

With that being said, I'd want a die that sized it down to .534" not .530". I don't have Lapua brass but maybe someone can chime in on what the head diameter is on new Lapua.

 

[Mahziar]

One more dimension to consider is how far that throat has advanced. Find dimension to lands and see how your CBTO relates. Obviously your difference of .030" in your CBTO that you stated is addressing the advancing throat erosion. How much jump?

For all the tests I tried to keep 0.020 jump
The lands cbto was 2.872, for last loading session measured at 2.907 after 400 rds

 

[CharlieNC]

The presentation of the data is not clear to me, when was test two shot? On the final five shot results the average SD is around 8, meaning the initial single shot results may be within that distribution and you are looking at noise. But as I mentioned test two is not clear to me.

 

[L.Sherm]

So your sizing die does not match your chamber if it's sizing down to .530"? .005" is a lot of sizing of that area. What brand die? This may be part of the equation but things like an enlarged bore or advancing throat erosion make more sense.

Hes got the AW2 chamber Alex designed, there's nothing wrong with his sizing, you gotta have it to stay away from clickers just like the 20VT with Lapua brass.

 

[XTR]

You’re assuming that your chrono data is repeatable Between range trips.

 

[L.Sherm]

With that being said, I'd want a die that sized it down to .534" not .530".

Not on a PRC and some others with ADG and Lapua brass unless you want bad clickers

 

[LCazador]

Hes got the AW2 chamber Alex designed, there's nothing wrong with his sizing, you gotta have it to stay away from clickers just like the 20VT with Lapua brass.

The 20VT Lapua is larger at the .200" line than the old chamber size so yeah you need to size the brass down to fit the standard 20VT chamber. We're talking about .005" smaller here. You're talking about apples and oranges.

 

[LCazador]

For all the tests I tried to keep 0.020 jump
The lands cbto was 2.872, for last loading session measured at 2.907 after 400 rds

Well that certainly is within the parameters of your tests. The downward trend on velocity is a mystery. I've experienced this before but I solved my problem with new brass and a completely different load.
That brass had an enlarged head diameter compared to the chamber and clickers are a problem when brass diameter at the head exceeds chamber diameter. Good luck to you solving the problem.

 

[Ned Ludd]

I've looked through your original post several times and am still struggling to develop an image in my mind of what is happening with velocity over time. My suggestion would be that you select a specific charge weight that you have used throughout the load development process, then illustrate graphically exactly how the velocity has changed over time, including any changes to the load during that process such as COL. This will make it much easier for members to provide useful feedback in response to your original question.

In more general terms, apparent changes in the average velocity derived from a 5-shot group of 10 fps, 20 fps, or even as much as 30 fps can occur via different mechanisms. Not all of the apparent changes are even real in that the use of a small sample size can make it seem as though average velocity is changing over time when in fact it is not. Other seemingly small occurences can also affect velocity recordings such as altering seating depth, changes in the brass (i.e. virgin vs fire-formed, annealed or not, etc.), chronograph effects, and environmental factors. It is not uncommon to observe the average velocity of an optimized load [apparently] change by as much as 5 to 10 fps on different days due to an effect of even one or two of these factors. Thus, it is possible in more extreme cases to observe even larger [apparent] changes in velocity from a combination of several factors. My point here is that it's difficult for readers to assess exactly what is happening with respect to the magnitude of the velocity change from the original information provided in the OP. For example, day-to-day changes in [apparent] average velocity that are not markedly greater than the ES/SD values are expected when using a relatively small sample size. This could be what's happening with the OP's load, but it's simply hard to tell from the information provided.

 

[L.Sherm]

The 20VT Lapua is larger at the .200" line than the old chamber size so yeah you need to size the brass down to fit the standard 20VT chamber. We're talking about .005" smaller here. You're talking about apples and oranges.

No I'm not look at the saami reamer and compare to Alex's.
Tod kindlers original was based on 221 Fireball brass, when Lapua came out well clickers happened untill the .200 line was opened up same thing with the PRC's.
I had to do the same thing on my 30-28 using ADG brass.
Pull up Kindlers 20VT print then compare the JGS 20VT lapua print.
Besides O.P says his chamber is .535 his fired brass is 5325-533 and sized is .530.
My guess not many firings on it yet when it work hardens a little more with spring back it will be more like .531 at the web which is exactly what most get with sizing.

 

[Mahziar]

The presentation of the data is not clear to me, when was test two shot? On the final five shot results the average SD is around 8, meaning the initial single shot results may be within that distribution and you are looking at noise. But as I mentioned test two is not clear to me.

Test 2 was next day after test 1

 

[Mahziar]

Ok, just got my kestrel D3
The bottle that was open a few days ago and was perfectly sealed measures at %35.8 RH which is right about my garage RH, and I just opened another bottle of Retumbo and it shows about %42 RH
That reloading night I opened a new bottle
And god damn Retumbo I can’t find any 8LB jugs. I don’t know if this %6 difference is part of the reasons of losing velocity from freshly opened bottle to the bottle that was opened a few days ago

 

[moondog]

Dry powder will be faster, so no