Discussion Forums

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:
50800cup:
(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.

Charlie/.308guy
[email protected]