Converting CUP to PSI

phishroy

phishroy

CGN Regular
Rating - 100%
102   0   0
Location
Durham region
Im working on a new load for some cast 243 win’ and i am considering IMR 7828 as a powder for several reasons which are complicated and i don’t want to explain at this point

The bullet is 99grain triple lube groove gas checked wheel weights water quenched at a BHN of around 16 BHN which means for me that I can push it to a max of 20433 PSI according to the lee reloading manual on page 134.

Using the lee manual it explains how to extrapolate jacketed bullet load data and reduce it to lead bullet working pressure , as long as some rules are fallowed which are indicated in the manual.


Hers is the kicker....
When looking up the info in the manual for IMR 7828 it lists the units in CUP and not PSI.

Here is the info’ from the manual (*pg 271)
IMR 7828 MAX 46 grains = 3009fps 51100CUP -1 grain factor velocity= 0.9791
and pressure =0.9563

I know that there is no conversion of CUP to PSI since these are different measurements all together but does anyone have a clue how to approach this matter in figuring out how to down step this load?
 
As I understand it the old CUP method of pressure was not a linear unit so I don't think there is any way to accurately convert it to PSI. There may be some more technically oriented members who could shed more light on this.
 
Now you're going to increase the pressure? To what - 40K? That powder will need at least that, or it will be erratic, sometimes just fizzle and often leave a bullet in the bore. You need to be very careful with reduced loads with medium to slow powders, best to use something in the SR4759 or faster range if you want a good 20K CUP (or psi) load.
 
Oh , I knew this would stir the pot, didn’t intend too.
Thank you for the heads up on the powder burn.

My intentions are to use a slow burning powder with a buffer I know there is lots of controversy about it.
Iv seen lots of information about using reduced loads of slow burning powder with buffers and how it actually increases accuracy at high velocity in cast bullets.
Im still researching into it and im trying to get more information and actual numbers.
 
Last edited:
phishroy said:
Oh , I knew this would stir the pot, didn’t intend too.
Thank you for the heads up on the powder burn.

My intentions are to use a slow burning powder with filler I know there is lots of controversy about it.
Iv seen lots of information about using reduced loads of slow burning powder with fillers and how it actually increases accuracy at high velocity in cast bullets.
Im still researching into it and im trying to get more information and actual numbers.
Click to expand...

The point I was trying to make is that each powder has its own range of pressures in which it will burn well. Fast powders burn well from very low to the very highest pressures. The middle to slow powders have a bottom limit under which they simply will not perform. The formula you provided is a roung rule of thumb that only applies (roughly) in some pressure ranges. For example, with IMR7828, it might apply from 70K down to 40K, then it ceases to produce expected results. IMR7828, at 30K for example will produce at best erratic results and lots of unburned powder - below that you'll likely get no more than the primer pressure and a glob of partially and unburnt powder. From experience.

See for yourself, and then buy some SR4759, Unique or the like.
 
7828 is what i use in my 338 win mag- i wouldn't even CONSIDER it for 243, which is just a 6mm/308- far TOO SLOW burning , let alone throwing a lb into the mix, which means even lower velocities
 
"...high velocity in cast bullets..." Too much velocity causes leading.
The cast loads, all with gas checks, in my Lyman book are shotgun powders. All velocities are under 2000fps.
 
sunray said:
"...high velocity in cast bullets..." Too much velocity causes leading.
The cast loads, all with gas checks, in my Lyman book are shotgun powders. All velocities are under 2000fps.
Click to expand...

I have safely been able to push my 243 to 2500fps with just under MOA accuracy at 100 yards with cast bullets WW water quenched and gas checked and no leading even after 150 rounds.
i dont use shotgun powders for my high velocity lead loads , pressure spikes to high to fast.

It took some research time and allots of calculating and double checking but it is doable.
Im looking to push the envelope a bit further but im not going to compromise on safety.
Im doing my homework
 
I always take to heart, every such thing about reloading that Andy gives.
However, reloading often seems to be more of an art than a science. Some things that should work don't, while something else that hasn't a chance, works out fine.
Having said all that I once did a lot of playing around with cast bullets in a 30-06. I poured my own and since I had access to linotype, that was what I usually used, and it is hard. I used every type of powder I had on hand and experimented with various lubes and bullet diameters. Regarding lube for the bullet, nothing seemed to change much, one apparently being as good as another. I also used different molds for bullets.
One of my most consistantly accurate powders was H4831! 43 to 45 grains of H4831 was as accurate as any other powder and better than most. This loading would be in the area of 70 to 75% of full load in a jacketed bullet. Too much of any powder created flyers. I was the only one shooting at this range in the winter with cast, so in the spring I picked up many of my bullets, undamaged by the winters snow. It was easy to spot the flyers. A groove, about as large as the wood of a kitchen match would be cut from end to end of the bullet. This escaping gas, of course, caused the bullet to wobble all over the place. All my bullets were gas checked.
 
I too have driven cast bullets pretty fast but usually find the sweet spot for accuracy, at 1800 to 2100 fps. I've had great luck with reduced loads and med. burning powders and reduced loads with fast burning "rifle" powders, but not slow burning powders, which i find quite a few "off" sounding shots. CBA benchrest cast bullet shoot results show the loads used and in most cases they use .30 cals. and reduced loads of med. burning powders , such as Varget, 4895, VV133, etc.
 
You must log in or register to reply here.
Back
Top Bottom