CUP formula

phill21 · Posted: Fri Aug 01, 2008 6:00 am Post subject: CUP formula

OK here is question that has probaly been brought up before but i can't find it.
So here goes is there a formula to work out your handloads CUP for a 243 if so what is the formula PLEASE???? Sad

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Using winchester brass, winchester 9 1/2 primers WIN 760 powder 85gr Seirra HPBT with an OAL of 2.585" and velocity of 3242 FPS. Sniper

I know this is at the max recomended but if i creep it up i need to be able to work out pressures as yet there are no pressure signs on my brass. Help

phil

SingleShotLover · Posted: Fri Aug 01, 2008 6:29 am Post subject: Re: CUP formula

According to Hodgdon, your loads with 760 will run from roughly 41,400 CUP (42 grains) to 50,200 CUP (45.0 grains) using a Barnes bullet. No idea where that places you with the Sierra though. I am not aware of a formula to figure it out, but I'm sure someone else has one.

RePete · Posted: Fri Aug 01, 2008 6:36 am Post subject: Re: CUP formula

They are now using PSI instead of CUP.

chambered221 · Posted: Fri Aug 01, 2008 9:17 am Post subject: Re: CUP formula

Unless you’ve got pressure testing equipment you’ll never know the actual CUP/PSI of your load.
It is possible to max out on your guns/load pressure before reaching the advertised limits.
It's also possible that your components are not giving you the pressure the book says they should give you.
There are way to many variables involved that influence pressure to form accurate data from calculations alone.

This is why they say “start low and work up”

If you feel your combo has more in it, by all means add another .2 or .3 at a time and see how it goes.

Just be sure you know what to look for and how to interpit it !!!!

Bushmaster · Posted: Fri Aug 01, 2008 12:57 pm Post subject: Re: CUP formula

"They" are still using both CUP and PSI...You just have to diferenciate between them when posting pressures.

English Mike · Posted: Fri Aug 01, 2008 3:14 pm Post subject: Re: CUP formula

Lyman 48th lists 44.5gr as MAX with the 85gr Sierra HPBT, a velocity of 3176fps & a CUP of 50,800.
It's listed as a compressed load.
Rifle used: Universal receiver & 24" 1:10 twist rate barrel.
Cartridge OAL 2.615"

phill21 · Posted: Fri Aug 01, 2008 3:37 pm Post subject: Re: CUP formula

OKKK! looks like i am already pushing the limits thanks guys. Hiding

phil

gelandangan · Posted: Fri Aug 01, 2008 5:19 pm Post subject: Re: CUP formula

Phil mate,
to achieve 3242fps your load would be about 46.5gn (94.4% case capacity) and according to calculation with standard 20 inch barrel is at 67,138psi and that is way over the safety limit of 60,000psi. Shocked

On the other hand, by increasing your OAL to 68mm (2.677inch) and reducing your load to 45.5gn (92.3% capacity) the pressure came down to 59,785psi (just inside safety limit) but your velocity is still at around 3150fps mark.
You will get almost the same velocity at a lot more lower pressure.

Eitherway, just by increasing your OAL by 0.1 inch, you are already lowering your pressure. Just beware NOT to increase OAL too much so that the projectile touch the rifling - if this happen, you are then increasing your pressure by another 8000psi <-- D A N G E R

Mind you I have done this theoretical calculation without knowing your gun, so you may have to tune it to suit your gun.

Vince · Posted: Fri Aug 01, 2008 6:01 pm Post subject: Re: CUP formula

Phill...what Chambered221 says is correct...without pressure testing equipment, strain gauges you will never be able to work out what pressure your loads are peaking at.

Just out of idle curiosity mate...why do you want to push the envelope beyond the listed maximum? Having been a bit of a velocity freak in days gone by, I have been there, done that, and suffered the consequences of damage to my firearm...all in the name of mega FPS and "manly" recoil.

My suggestion is that you do some research on powder/projectile combinations, select a load, then do your load development again to achieve that which you are chasing.

If its accuracy you are chasing, then you will be surprised to find that the optimum accuracy will arrive before you reach the maximum load. As an example, using AR2208 and Remington 80gn PSP bullets my .243 is most accurate at .5 gn below maximum.

If its velocity you are chasing then you need to look long and hard at the powder you use and the weight projectile you use. Accuracy and safe pressures are not necessarily going to be a part of the equation.

Read up on the methods of obtaining good accuracy found in the articles on the Home page of this Forum....its well worth the trouble.

Cheers, Vince

phill21 · Posted: Sat Aug 02, 2008 3:54 am Post subject: Re: CUP formula

Hey Vince it was sort of idle curiosity to see if there was a formula to work out the PSI and still keep it with in safe limits. We all know that every rifle, handgun is different so i thought that as such the reloads would have more flexibility.

The reload didn't exceed the maximum and i thought i would check with those who know before doing anything to silly.

Thanks guys Very Happy

phil

Vince · Posted: Sat Aug 02, 2008 4:10 am Post subject: Re: CUP formula

No mate, no easy equation to find out pressure. With strain gauges it is easy...if you know how to use them. Thankfully the manufacturers of powder and bullets have done the hard work for us and provide the pressure level of a particular load/bullet combination...not that it means a great deal, so long as we remain within the parameters of the load data.

Cheers, Vince

ElyBoy · Posted: Sat Aug 02, 2008 8:59 am Post subject: Re: CUP formula

I would give Vince's advice serious consideration Phill.
I have had only one rifle or pistol over the years that gave me good accuracy towards the top of the pressure charts, and that is my 300WinMag.
Even with this rifle, I worked the load up slowly from the low pressure, and even then, my top accuracy load is much lower that the max.
Tread lightly, and work up slowly when working a load.
You will be amazed that most of the time, your top accuracy will be closer to the lower pressures than the higher pressures.

Using this method, your rifle, fingers, hands, and face will remain in one piece.

Eric

dmickey · Rookie Member Joined: May 11, 2008 Posts: 1

What's wrong with loading for accuracy instead of velocity? I'm sure the game won't notice a slight decrease in velocity!

Bushmaster · Posted: Sat Aug 02, 2008 10:22 am Post subject: Re: CUP formula

WOW!! Right out of left field... Very Happy

Aah...Welcome to this fine site, dmickey...I'd ask ya to pull up a stump and grab a beer, but ya already have...Jump right in... Very Happy

No one saying to load to maximum, only saying that if you do...Be careful. That said...I load .357 magnum near maximum loads of W296 and .30-30 just under maximum load of W-748. As a matter of fact I'm just under maximum load of H4831SC for my .30-06...But you are correct. It isn't necessary...And the game won't no the difference.

Again...Welcome to this fine site...

chambered221 · Posted: Sat Aug 02, 2008 10:32 am Post subject: Re: CUP formula

dmickey wrote:

What's wrong with loading for accuracy instead of velocity? I'm sure the game won't notice a slight decrease in velocity!

They won't notice the extra FPS evryone tries to get either !!! wtf

Welcome to the site dmickey Howdy