Is interpolation of loads safe?

[schuetzen]

Question: What are opinions on the safety of interpolation of load data to derive unknown powder loads?

Below is an example:

Speer Manual (data for Speer .308 100gr "Plinker" bullet)

.308 WIN data
Powder  (start)  -->  (max)
2400        ?       -->   ?
H322    44.0 gr  --> 48.0 gr

.307 WIN data
Powder  (start) -->  (max)
2400     20.0 gr --> 24.0 gr
H322    31.0 gr --> 35.0 gr

If I interpolate using this data I come up with...

2400     28.4 gr --> 32.9 gr

Similarly using other powders for interpolation the results are:

Interpolated from IMR 4895 powder..
Powder  (start) -->  (max)
2400     24.4 gr --> 28.8 gr

Interpolated from 748 powder..
Powder  (start) -->  (max)
2400     25.3 gr --> 29.7 gr

I would conclude from this that it should be safe to start with 24.4 gr of 2400 powder for the Plinker bullets.

[stimpylu32]

It is NEVER a good idea to simply use the WAG ( wild a-- guess ) method to come up with load data , there are just too many things that need to be taken into account , case cap , burn rate , pressure range of the powder , pressure range of the cartridge , the type , age ,  action of the weapon .
 
A far better choice is to call the powder company and ask the true experts what they say if no reliable published data is to be had , its not worth the danger to simply guess .
 
stimpy

[huntducks]

I don't even know what that word means (interpolation) but if your asking for a starting point for 100gr plinkers in a 308 I can tell you I use to use them in a 300 savage md 99 for jackrabbit hunting all the time and it was 40grs of 4895. and in a 30-30 it was 30grs they looked like rags when hit.
 
I don't think 2400 would be my powder of choice.
 
I think my older Speer manual has a bunch of loadings for the 100gr plinker i'll look when I go into my loading room.
 
 

[stimpylu32]

I have never found a 1 to 1 for any given powder , by that I mean that if 20g of X powder and 30g of Y powder is ok for a load in one round , then then 25g of X powder and 35g of Y powder is ok for another round . it just don't work that way .
 
stimpy
 

[quickdtoo]

I'd contact Alliant for 2400 loads, they listed it in their 2005 guide for the 150gr Sierra.

Tim

http://www.alliantpowder.com/questions/default.aspx

[schuetzen]

I have never found a 1 to 1 for any given powder , by that I mean that if 20g of X powder and 30g of Y powder is ok for a load in one round , then then 25g of X powder and 35g of Y powder is ok for another round . it just don't work that way .
 
stimpy

I've posted this more of as a curiosity.  I can solve my problem by purchasing another powder.  However, I can't identify any other variables not allowing one to do this.  The only difference is the differences in the brass case, in this situation between a .307 WIN and a .308 WIN.

****************************************************
Given the information from the Speer manual:

.308 WIN data
Powder  (start)  -->  (max)
2400         ?     -->     ?     <-- Information desired but not listed in Speer manual.
H322    44.0 gr  --> 48.0 gr

.307 WIN data
Powder  (start) -->  (max)
2400     20.0 gr --> 24.0 gr
H322    31.0 gr --> 35.0 gr

Same bullets (Speer .308 100gr "Plinker" bullet)
Same primers (CCI 200)
If same .30 cal barrels are used (the difference being the chamber for the case)
Excluding the use of semi-auto rifles, firing strictly from single shot barrels.

I can think of no other variables that would not allow this to be a viable (estimation) of starting loads.  The only true variable is the brass case design.  In this case the .307 WIN is virtually identical to the .308 WIN in this case.
****************************************************

Unknown: What powder charges (start) and (max) could be estimated from known data above:

Start 2400 powder charge calculation
(20.0 gr / 31.0 gr) X 44.0 gr = 28.4 gr

Max 2400 powder charge calculation
(24.0 gr / 35.0 gr) X 48.0 gr = 32.9 gr

[schuetzen]

I don't even know what that word means (interpolation) but if your asking for a starting point for 100gr plinkers in a 308 I can tell you I use to use them in a 300 savage md 99 for jackrabbit hunting all the time and it was 40grs of 4895. and in a 30-30 it was 30grs they looked like rags when hit.
 
I don't think 2400 would be my powder of choice.
 
I think my older Speer manual has a bunch of loadings for the 100gr plinker i'll look when I go into my loading room.

Interpolation is using known data to derive unknown data.

[stimpylu32]

schuetzen
 
In this case ONLY , the data I would use is the data for the 307W , just for the fact that it is basicly a rimmed 308 but works at a lower pressure , now I would not use the data for the 308 in a rifle made to shoot only the 307 Win cartridge , that could be BAD . 
 
My first responce was a more general statement , for a rule of basic loading safety .
 
stimpy

[huntducks]

schuetzen
 
In this case ONLY , the data I would use is the data for the 307W , just for the fact that it is basicly a rimmed 308 but works at a lower pressure , now I would not use the data for the 308 in a rifle made to shoot only the 307 Win cartridge , that could be BAD . 
 
My first responce was a more general statement , for a rule of basic loading safety .
 
stimpy

+1
 
There is no way I would use data from another Caliber even if the case was a necked down or up verision it would be same as using 308 data in say a 243 you could get away with it with slow burning powder but your asking for trouble with faster burning powders.

[huntducks]

Are you looking for 308 loads for the 100gr plinker I have a Speer #7 manual in front of me which list a 1/2 doz different powders for 100gr plinker.
 
Let me know if you want any info.

[schuetzen]

Are you looking for 308 loads for the 100gr plinker I have a Speer #7 manual in front of me which list a 1/2 doz different powders for 100gr plinker.
 
Let me know if you want any info.

That's OK.  I have the Speer manual also.  I have 2400 powder on hand, forcing the question.

I am looking into using another powder that I can use broadly with .308 WIN.  I think I may go with IMR 4064.  It is in most of my loading manuals for .308 WIN and uses CCI-200 primers that I already have.  I was also considering WIN 748 which appears to be widely used, but my manual calls for CCI-250 primers which I don't have.

[LanceR]

H 4895 can be used down to 60% of the max charge for any cartridge for which it is listed.  Look under the "DATA" tab on the Hodgdon home page for info. 

Lance

[Dand]

Schuetzen - from your original post it was not at all clear what you were trying to do. Guesstimate a 308 load from 307 data or visa versa. And in that case Stimpy was right on in his first post.


Reading thru the whole thread your last post makes some sense but your initial  interpolation system scares me still. I'd go to the Lyman book and look at their cast bullet loads where they probably list 2400.  Since I believe the plinkers are a half jacket, I'd start with the starting loads for a 100 gr cast bullet and work up carefully.


Or even better, like LanceR says - 4895 is very forgiving and can be downloaded to a great degree.