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Thread: cup and psi

  1. #1

    cup and psi

    i know what each stands for but i as read lee modern reloading some reloads are in cup and some in psi , what is the relation between the two ; is cup 15 % lower than psi or is it a rising scale ? what are max pressures for .243 and .308 , not that i want to go near max pressures but want to choose a good powder that provides a good balance between the two (pressure and velocity)...looking at lees book h blc-2 provides good velocity and lowish pressure in .308 125 grn it looks about 7-8000 psi lower than other powders so would this be a good powder to start with? thanks sb.

  2. #2
    Sorry but I have no idea how they relate to one another. Lee uses other peoples information and puts it in one place which is very useful. Now BLC-2 is an excellent powder in .308 but ........................... but one thing to beware is that it often provides a quite spectacular muzzle flash. I use BLC-2 with the 130 grain Hornady in the .308. I don't know what velocity I am getting but the accuracy is excellent.

  3. #3
    There is no hard and fast relationship between the two, really.

    Loading to pressure is a good idea. I do it all the time.~Muir

  4. #4
    Mixing reference points and data is sadly a recipe for disaster, if in doubt , use one set.
    (The Unspeakable In Pursuit Of The Uneatable.) " If I can help, I will help!." Former S.A.C.S. member!

  5. #5
    brit thanks for previos info on ph rifles but ive just bought a krico 600 in .308, when you say spectacular muzzle flash .. i dont want to set the forest on fire or singe my eyebrows how bad is it.
    muir..i was thinking of a good balance between velocity and pressure but dont know how to compare different powder in cup and psi . thanks to both for replies sb

  6. #6
    Quote Originally Posted by satellite buck View Post
    brit thanks for previos info on ph rifles but ive just bought a krico 600 in .308, when you say spectacular muzzle flash .. i dont want to set the forest on fire or singe my eyebrows how bad is it.
    muir..i was thinking of a good balance between velocity and pressure but dont know how to compare different powder in cup and psi . thanks to both for replies sb
    Actually the shooter does not notice it much by bystanders do. First time I touched one off ont he range it grabbed the folks attention. Several asked what I was shooting. They now ignore it in low light it will certsinly show up. I suggest you try a few loads and see what you think. After all accuracy is superb. If I do my part it will put three shots touching.

  7. #7
    CUP and PSI is a bit like measuring temperature in Fahrenheit with an old fashioned mechanical capillary gauge and then comparing the results to a digital thermometer reading Celsius. As a bit of a mathematician I feel compelled to point out that two things attempting to measure the same property must have some kind of a relationship, however that relationship may not be linear and there will be variance owing to errors in measurement methods.

    Here's an interesting article on the subject: http://www.shootingsoftware.com/ftp/psicuparticle2.pdf

    Practically speaking though, as has been said, this is a guide only and shouldn't be used to replace safe and conservative reloading practices.

    Alex

  8. #8
    Well Put! It's a different kind of measuring system. The CUP is usually lower than PSI because of the method of measurement: A copper crusher/piston arrangement VS a piezoelectric reading. WM Davis of the NRA (US) technical staff places CUP at 5 to 20% below PSI, BUT depending on the shape of the case and other situations they encountered, it could be higher.

    In other words, there is no exact correlation between the two. Remember that SAAMI for many cartridges were established by the crusher method in so measure in CUP. If you see 54,000CUP as a Maximum with 45 grains of Powder "X", that is still your maximum load. Look around for other lab data if you want PSI but it won't change anything. That will still be the top charge. ~Mui

  9. #9
    thanks for the replies, am not trying to increase max loads in wieght or pressure. i was trying to compare two different powders for pressure but one was in psi and the other in cup, will always begin with start weight and work up.have read in lee mrl 2 to try to find a powder to fill much of case as you can but still be under max pressure, to this effect i have been looking at the data for h blc-2 and relaoder 12 both give good case fill and stay under max pressure and give good velocity. i note that blc-2 has a start of 48 grn-upto 52 grn , reloader 12 start at 46.6 upto 49 (page425 lee mrl2/125grn) so would blc-2 be a better powder to start with as a beginner(reading a previous post by muir about weight and volume). any thoughts chaps. sb

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