Folkestone Engineering Supplies
To proof or not to proof?
I thought it may be useful to show a cartridge proof report and what it tells us about are home loads.
This lot of ten cartridges was tested late in 2015 (Due to some commercial confidentiality I have blanked out the powder type and BPH number and date).
So what can we learn from the report?
(1) You will notice that the cartridges are tested at a CIP standard temperature which is shown below the Test Reference Number.
(2) The test barrel used will also conform to the CIP rules for proof, e.g. minimum bore diameter for said gauge etc.
(1) and (2) above ensure we always compare like-with-like and hence to a known standard - CIP.
The first table shows the shot number (1 to 10) and each shots velocity at 2.5meters in meters/sec, Momentum (recoil) and pressure in bar (P1 being the pressure at 1" in the chamber) measured by piezo transducer.
The second table shows the summary results based on the ten shots.
So for these fibre wad cartridges the mean (or average) velocity was 397.91 m/s at a mean pressure of 615.97 bar
Spot on so far, nice average speed and sensible average pressure.
They go bang, but are they safe to use and are they ballistically a quality cartridge?
CIP define the limits of average
pressure for the 12ga depending on chamber length this is known as Ptmax.
They also define statistical maximum pressure Pn <= Ptmax and Pn+(K2n x Sn)<= 1.15 Ptmax which is the maximum pressure that no single cartridge must exceed.
These important results are shown under Statistical Analysis and for this ten cartridges the results were:
Pn+(K2n x Sn)<=1.15 Ptmax gives 681<=851bar
And against each statistical result "CIP Conformity" the important result Conforms (or could be Fails).
So these cartridges conform to CIP standards and are safe to shoot, however are they a quality cartridge?
This is where within the Summary table the Std Dev (standard deviation) and Max-Min are your guide.
The lower the std dev and max-min the more consistent the cartridges are i.e. for each shot velocity and pressure will be similar to each other.
It is possible to have very good std dev on the velocity and very poor std deviation on the pressure.
Plastic wads in my experience are far more forgiving on issues that effect the Std dev like, crimp pressure, powder type and dose variation, Fibre wads are generally more challenging and give a larger Std dev on pressure than plastic wads.
I would say with a Std dev of 5.94 (max-min 18.68) for velocity and 22.21 (max-min 59.32) for pressure that these are not only a safe cartridge but of good quality for a fibre cartridge.
By comparison the same load but with a different powder, conformed to CIP but gave
Velocity std dev=10.53 max-min
24.10 ---- reasonable
Pressure std dev=56.22 max-min 128.90 ---- poor
and hence even given they conform to CIP and are safe to shoot that they are not as consistent a cartridge as the first lot.
It is this consistency that separates out a good cartridge from an average one.
Only by sending cartridges to proof
(unless you are lucky enough to have your own test equipment) can you obtain
Also velocity on it's own is not a good indicator of pressure, you will notice that the highest velocity in the ten shots was not also the highest pressure.