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returning 7mm wsm load info to you

07 Jul 2013
@ 01:00 pm (GMT)

charlie porter

returning 7mm wsm load development

hi Nathan

ive been locked out of my email account since last nite and just got back in- noticing your email to me concerning rifle loads.

first off- ill need full case capacity in grains for your .280 al to help me with that cartridge. please send.

next-for your 7mm wsm using your following given data:
bullet 162 gr/ 1.43 lg
full case cap 77.561
barrel lgth 24
coal 2.944

at a load density of 86% for initial moderate accurate starting load =58.6 gr h4831
approx. mv 2642 fps
approx. pressure 42000 cup
expansion ratio 6.2
approx. max load 64.8 gr / mv 2921 fps /50800 cup pressure

Nathan as you know- cup units differ from psi units but a fairly accurate conversion from cup to psi is: psi=-17902+1.516cup. so lets calculate your moderate starting load of 48000cup into a psi figure:
(1.516 x 42000) - 17902=63672-17902=45770 psi.
so in summary your moderate accurate starting load in pressure breakdown is:
42000cup/ 45770psi
next: approx. max load figures are:
(50800 x 1.516) - 17902=59110 psi which is close to 62000psi limit on non magnum cartridges.

so in summary max load :
50800 cup
59110 psi

knowing that the approximate safe limit on non magnums is 52000cup and on magnums is 54000 cup these translate into psi limits of 61000psi and 64000psi respectively.

I take it that your 7mm wsm is a short magnum Winchester so your pressure limits could go up safely a bit more based on how ive explained things but then case life decreases and throat/bore erosion increase accordingly as you well know and bullet stress increases causing potential erratic performance in the field.

I take it you might think of me as a moderate high pressure kind of guy as I place emphasis on long case and barrel life. as an m14 service rifle shooter for 15 years I shot the 7.62mm m852 round 168 gr sierra using 41.5 gr imr4895 at a pressure of 50000cup from 200-600 yards over the course and after 15000 rounds still had accurate barrel life to 600 yards at approx. 1 moa.

I enjoy developing moderate pressure starting loads for shooters and can also calculate corresponding trajectories if I know the bullets form factor and sectional density which is easy enough to obtain from bullet dimensions.

I must say I really enjoy corresponding with a guy like you who really knows his business. I hope I can continue to learn from you and look forward to having you test my bullet designs when the time comes.
one final note- as you obviously know its always best to have actuall full case capacity for the actual brass case your using. for your 7mm wsm I used a figure that is merely representative -but your particular case capacity may be higher or lower slightly altering my calculations- but at least you understand. feel free to send me your actual case capacity(full in grs) so I can adjust my figures for you accordingly.

[email protected]


07 Jul 2013
@ 01:33 pm (GMT)

charlie porter

Re: returning 7mm wsm load info to you
youll see an error in my reply to you concerning 7mm wsm.
the error occurred when I wrote"lets calculate your moderate starting load of 48000 cup into a psi figure. it should have been 42000cup.
however all the calculations are correct and based on 42000cup as youll see.

07 Jul 2013
@ 04:51 pm (GMT)

Nathan Foster

Re: returning 7mm wsm load info to you
Thanks Charlie, that was pretty good for a computer generated estimate. I wanted to test your program because I am hesitant about you offering advice on this forum without first hand experience with a cartridge. Hence why I emailed my questions to you as opposed to posting them. But what you have done so far looks pretty good.

My start load for the WSM was 62gr, velocity 2795fps.
64.8gr gave 2850fps, about 70fps off your estimate which is not too bad.
Sweet spot 67.5gr in that rifle for 2960fps (2960fps being a typical sweet spot)

In another rifle, best accuracy was at 67 grains for 2975fps.

In yet another rifle using the 162gr SST, the sweet spot was 63.5 grains for 2860fps. The barrel was too thin and whippy to be accurate with a stouter load.



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