How is CUP calculated?

[fr3db3ar]

I'm loading some hot loads for my .357

With a 358 bullet their max load says it's max load is around 22000 cup

with the 357 Hornady bullet...they're max load generates 40000 cup

I'm going over the max of the 358 bullet but should still be below 40000 cup....if I knew how it was measured.  I'm guessing my load will be around 31000

How can I know for sure without guessing?

[wncchester]

You and we can't.  It's measured, not calculated.

[charles p]

Always wanted to know.  How the heck do they measure it?

Wife says I've been in my cup before. 

[billy_56081]

I believe thye use a barrel with a hole drilled in it. In the hole there is a copper pellet placed in and a threaded screw put over it to hold it there. It is measured before and after the gun is fired and then these measurments are used to calculate pressure. Now pressure in most often measured with electronic pressure transducers and is givin in PSI.

[maddogg]

I hope this helps.

Perhaps the greatest area of misunderstanding of pressure terminology, has arisen from earlier practice of referring to chamber pressure as some number of "pounds per square inch (psi)". Many earlier ballisticians believed that results of a firearm chamber pressure tests were accurately described as psi. The truth was their test results had little to do with actual pounds per square inch.

Their "crusher" testing and the testing still used in many ballistic labs, uses a barrel with a hole drilled into the chamber perpendicular to the bore's axis, usually 1" from the rear edge of the chamber. A slip fit piston is fitted into this hole with its end contoured to precisely fit the inside of the barrel chamber. In testing, a cartridge is loaded into the chamber and the piston slipped into place. A copper crusher is then stood on top of the piston and is securely held in position with an anvil. When fired, the cartridge case has a small disc rupture from it at the location of the piston hole. The hot and rapidly expanding gasses in the chamber push equally on the bullet base and on the piston base. The piston in turn moves heavily on the copper crusher, which is forced to collapse to a varying degrees depending on the total amount of pressure applied to it by the piston.

The amount of "crush" of the copper cylinder is then measured carefully and this crush length is compared to a tarage table which lists a specific value for the amount of crush which occurs.

There is, of course, more to the crusher pressure testing system then the foregoing few words might suggest. The point to be made is that the copper (lead for shotguns and some handguns) crusher method is a valid and useful tool for ammunition evaluation, but it does not actually express pressures in true pounds per square inch. This did not escape the attention of newer generation ballisticians. They set out to correct the misnomer of pounds per square inch and used instead the designation copper units of pressure (CUP's) or lead units of pressure (LUP's). However, the erroneous term PSI had become so accepted that it was frequently used interchangeably with CUP or LUP. While such use was technically wrong it created no major problems when everyone was talking about the same thing; the result of a copper or lead crusher pressure test.

[TribReady]

Very interesting, and informative, don't get me wrong, but...

I thought this topic would be a lot different and a lot more "interesting"         

[billy_56081]

Very interesting, and informative, don't get me wrong, but...

I thought this topic would be a lot different and a lot more "interesting"         

  I get what you meen.

[fr3db3ar]

Very interesting, and informative, don't get me wrong, but...

I thought this topic would be a lot different and a lot more "interesting"         

LOL......I'm just trying to ensure I don't blow up my revolver   lol

So...better measurements...and perhaps somebody can tell me if I'm way off.

Cartridge:      357 Magnum
Load Type:     Pistol
   Starting Loads
   Maximum Loads
   
Bullet Weight (Gr.)    Manufacturer    Powder    Bullet Diam.    C.O.L.    Grs.    Vel. (ft/s)    Pressure    Grs.    Vel. (ft/s)    Pressure         
158 GR. CAST LSWC     Hodgdon     HP-38     .358"     1.610"     3.4     796     12,600 CUP     5.0     1109     23,900 CUP          
158 GR. HDY XTP     Hodgdon     HP-38     .357"     1.580"     6.2     1108     33,700 CUP     6.9     1220     40,000 CUP           

I'm loading a .358 Hornady 158gr RN with 6gr.  Is there some way to correlate the pressures from the other load data to fit my load?

I'm trusting that 40k is the highest I should go...and I think I'm still below that.

[billy_56081]

I would imagine the load for the lead bullet you list was maxed out at that level because of leading issues.

[trotterlg]

How cup is measured is a very good explination, and is accurate as to how it is done.  PSI is now measured using modern strain gauges and other methods, there is no crediable way to convert PSI to CUP or CUP to PSI, they are two totally different ways of measuring things and cannot be correlated with each other.  About the only thing that can be said is that more of each equals more pressure.  Although probably not always true, the same cartridge and load, measured both ways, most likely result in CUP numbers that are lower than PSI numbers.  Larry

[LaOtto222]

I wish there was a way to measure pressure for the home hand loader, but the cost is just too prohibitive.

Now for a "short" discussion on home measuring of pressure. The best you can do is to consult several loading manuals and interpolate your starting loads and max loads and hope you are not generating any more pressure than is safe in your gun. If you consult many different manuals with the same relative components; you will find a wide variance in max loads. Slight differences in bullet diameter from one lot to the next, powder lot difference, primer lot difference, case lot differences, barrel inner diameter difference are just a few factors that influence pressure. For the home loader, a chronograph is probably the best (but not perfect) indicator of pressure we have available at a reasonable cost. Find load data fired out of a gun with a similar design (revolver vs revolver; or bolt gun vs bolt gun) and length barrel and do not exceed it's velocity or the load data given; which ever one is lower. In other words if the data gives 1450 fps as max velocity with a certain powder... then do not exceed that load of powder, even if it does not yield 1450 fps. On the other hand if you reach 1450 fps and still do not have the recommended max powder charge, stop at that point and call it good. By doing so, you can rest assured (well as good as can be expected) that you are not exceeding a cartridges designed max pressure. Sorry if I bored anyone, but it gets my thinking cap on and that helps ward off dementia.

Good Luck and Good Shooting

[stimpylu32]

Very interesting, and informative, don't get me wrong, but...

I thought this topic would be a lot different and a lot more "interesting"         

LOL......I'm just trying to ensure I don't blow up my revolver   lol

So...better measurements...and perhaps somebody can tell me if I'm way off.

Cartridge:      357 Magnum
Load Type:     Pistol
   Starting Loads
   Maximum Loads
   
Bullet Weight (Gr.)    Manufacturer    Powder    Bullet Diam.    C.O.L.    Grs.    Vel. (ft/s)    Pressure    Grs.    Vel. (ft/s)    Pressure         
158 GR. CAST LSWC     Hodgdon     HP-38     .358"     1.610"     3.4     796     12,600 CUP     5.0     1109     23,900 CUP          
158 GR. HDY XTP     Hodgdon     HP-38     .357"     1.580"     6.2     1108     33,700 CUP     6.9     1220     40,000 CUP           

I'm loading a .358 Hornady 158gr RN with 6gr.  Is there some way to correlate the pressures from the other load data to fit my load?

I'm trusting that 40k is the highest I should go...and I think I'm still below that.

There are some other things to take into account , first is the curve of the pressure that is being generated , as all powders react differently . Take an apple for example , slowly lower a 10 pound weight on it and the odds are pretty good that it will stay an apple . now take that that same 10 pound weight and shove it down on to the same apple as fast as you can , odds are pretty good that you will have apple sause .

I know that this is a very crude example , but you get the idea . powders all burn at different rates of speed and thus build pressures at different rates , with some reaching MAX pressure in a very short time , while others build it slowly over a longer time . other factors that come into play include the amount of surface resistance of a given bullet , the amount of neck tension in the case , or the amount of crimp used to hold the bullet . all can affect the curve of the pressure .

Another thing to consider is the amount of time the chamber is subjected to a given pressure , it may be rated for 40,000 CUP for 1 mili-second and be fine , but may not do so well with 30,000 CUP over say 5 mili-seconds of pressure .

My point is that while the load your using may be building 23,900 CUP with 5g of HP38 , it may very well be building a much greater pressure with 6g , as the data shows just a 1.4g increase in powder caused the pressure to climb a total of 11,300 CUP .

So even using strait line logic , your load could be somewere in the 33,000 CUP range or even much higher , well above what the data shows as MAX for that load .

stimpy   

[wncchester]

"I would imagine the load for the lead bullet you list was maxed out at that level because of leading issues. "

Highly unlikely.  Loading data is referenced to pressure and velocity, not leading.   Leading results from many things; alloy, hardness, bore condition and fit, the lube used, etc.  No way a book maker can take those variables into consideration and spoon feed us the "right" answers.