The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Guns, Politics, Gunsmithing & Reloading Reloading CUP to PSI formulaGo | New | Find | Notify | Tools | Reply | |
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From an article from somewhere, I forget now where I copied it, by Denton Bromwell: To convert CUP to PSI (1.516 X CUP) - 17,902 = PSI. Example: 45,000 CUP X 1.516 = 68,220 68,220 - 17,902 = 50,318 PSI And this formula can be reworked to convert PSI to CUP. The Hodgdon Reloading Magazine lists a lot of loads for the same bullet using different powders. For some loads they will use CUP for one powder, then PSI for the next listed powder. Using this formuls, the loads can be converted to whichever you prefer. NRA Life Member DRSS-Claflin Chapter Mannlicher Collectors Assn KCCA IAA | ||
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I always use 1.5CUP-18000. Simpler, and despite what you read it is accurate. | |||
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Attempts, like this, to convert C.U.P. to psi can be pretty consistent with straight walled cases. However, bottleneck cases are not so consistent. Matt FISH!! Heed the words of Winston Smith in Orwell's 1984: "Every record has been destroyed or falsified, every book rewritten, every picture has been repainted, every statue and street building has been renamed, every date has been altered. And the process is continuing day by day and minute by minute. History has stopped. Nothing exists except an endless present in which the Party is always right." | |||
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Don't know that it a true conversion. I thought it simple lined up the pressure plots. I know it has been discussed numerous times here. As usual just my $.02 Paul K | |||
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Thanks for the info. IHMSA BC Provincial Champion and Perfect 40 Score, Unlimited Category, AAA Class. | |||
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No formula or conversion !   | |||
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Wow! Great info here. Let's see: My .38 Special load is listed at 12,000 CUP. So, 12,000 x 1.516 = 18,192. 18192 minus 17902 = 209 PSI. So, my .38 Special is getting 800 fps and a really loud bang out of pressure approximately equal to that in a gas station air hose? And if cut the powder charge a half grain I could reduce it to 11,000 CUP -- then it would suck the bullet backward? | |||
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It doesn't work for pressure that low. But I find it useful for rifle pressures. | |||
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The purpose of mathematics is a decision not a number. The conversion formula fulfills its purpose quite well. Mike Legistine actu quod scripsi? Never under estimate the internet community's ability to reply to your post with their personal rant about their tangentially related, single occurrence issue.  What I have learned on AR, since 2001: 1. The proper answer to: Where is the best place in town to get a steak dinner? is…You should go to Mel's Diner and get the fried chicken. 2. Big game animals can tell the difference between .015 of an inch in diameter, 15 grains of bullet weight, and 150 fps. 3. There is a difference in the performance of two identical projectiles launched at the same velocity if they came from different cartridges. 4. While a double rifle is the perfect DGR, every 375HH bolt gun needs to be modified to carry at least 5 down. 5. While a floor plate and detachable box magazine both use a mechanical latch, only the floor plate latch is reliable. Disregard the fact that every modern military rifle uses a detachable box magazine. 6. The Remington 700 is unreliable regardless of the fact it is the basis of the USMC M40 sniper rifle for 40+ years with no changes to the receiver or extractor and is the choice of more military and law enforcement sniper units than any other rifle. 7. PF actions are not suitable for a DGR and it is irrelevant that the M1, M14, M16, & AK47 which were designed for hunting men that can shoot back are all PF actions. 8. 95 deg F in Africa is different than 95 deg F in TX or CA and that is why you must worry about ammunition temperature in Africa (even though most safaris take place in winter) but not in TX or in CA. 9. The size of a ding in a gun's finish doesn't matter, what matters is whether it’s a safe ding or not. 10. 1 in a row is a trend, 2 in a row is statistically significant, and 3 in a row is an irrefutable fact. 11. Never buy a WSM or RCM cartridge for a safari rifle or your go to rifle in the USA because if they lose your ammo you can't find replacement ammo but don't worry 280 Rem, 338-06, 35 Whelen, and all Weatherby cartridges abound in Africa and back country stores. 12. A well hit animal can run 75 yds. in the open and suddenly drop with no initial blood trail, but the one I shot from 200 yds. away that ran 10 yds. and disappeared into a thicket and was not found was lost because the bullet penciled thru. I am 100% certain of this even though I have no physical evidence. 13. A 300 Win Mag is a 500 yard elk cartridge but a 308 Win is not a 300 yard elk cartridge even though the same bullet is travelling at the same velocity at those respective distances. | |||
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ALF, Thanks for sharing the neat plots! Where did you find them? They are very close to what we learned when a buddy and I did a lot of effort to figure out how to convert. We ultimately abandoned the effort because the inconsistencies were uncomfortably large. | |||
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Source: I have to go look but I think its form various sources ( the original text of the LE Brownell research on firearm pressure) Now if ever there was a complicated subject in conventional firearms function it is the one of pressure measurement and pressure prediction! Even the "old" copper or lead crusher it by no means a simple process and anyone who thinks it is is deluding themselves ! The reason is self evident because for one we are dealing with the act of measuring ( a exercise in applied statistics) and secondly we perhaps underestimate what exactly we are measuring and its relationship to absolute pressure. The crusher gauge requires measurement of the stress and strain behaviour of a slug of metal in response to the application of pressure on its surface. It is by nature a complex process and interpretation of results require some understanding of what we are doing and trying to achieve. Pressure measurement and pressure prediction is part of the traditional ballistics problem and has been the source of much research ...... there are literally libraries of books written on it!  It is perhaps this very reason why I am sceptical of "reading" effects of pressure on a brass case by visual inspection and or measurements of dimensional change of a predictor of excessive pressure. When I see changes all that can be deducted is that pressure had an effect but how great that pressure was cannot be deduced other to say the case underwent change ! | |||
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Thank goodness I never pay attention to all this math for reloading! I start with a reasonable load, and work up. I stop when any sign of high pressure appears. I have had the same caliber guns vary so much that a starting load in one is excessive in another. Each one is is alaw unto itself. | |||
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Saeed: We all do that to a greater or lesser degree  My issue is not with the fact that we see that the case is showing change , my issue is when good folks try to convince me that measuring a point on a tapered cylinder to a scale of 4 decimal points on a metal object which we know is likely the least precise ( in terms of structure ) component in the internal ballistics event.... and then to claim that what they have measured is in any way accurate or precise in terms of how that case behaved under stress . Its like walking in the dark in a narrow path on the side of a precipice ! We know that the precipice is there but we have no idea of how close we are to it ! | |||
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ALF, good comments. Nonetheless, there may be some utility in measuring the brass at the same location before and after shooting. Getting the measurement in the same spot is a challenge and it must be at the web rather than slightly forward where one often sees a slight bulge even under normal pressures. The brass case and any permanent changes to the base during firing are analogous to the crush gage. It is a crude analogy to be certain. What is certain, however, that changes in case head diameter of more than 0.001" (.025 mm) (the detectible limit on most calipers) means that the stresses in the brass exceeded its yield strength. In other words, the brass is starting to flow. The rate of flow will depend on how much greater the pressure is than that needed to start the permanent set. So, that first indication of flow says there is a dangerous cliff nearby. The question for us handloaders is: "How close to the precipice do we want to go?" | |||
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Off course the brass reacts to pressure and it flows ! But the million dollar question is how to use that knowledge and even more importantly if what you are measuring a indication of impending doom  Those who work as professionals in the field of ballistics design say No.... we are reading tealeaves !  and they have put forth arguments to prove their point | |||
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You gentlemen do not have some of the geniuses we have here! Like tha chap who wanted to increase the power of his 12 gage shotgun shells. He discovered that there is a lot of empty space occupied by the plastic wad. So he opened a couple of shells, cut the middle part of the wad, added extra powder from the other shell, put the cut off parts of the wad in, added extra shot from the shell, and fired it in a Francchi semi auto shotgun. The rear end of the bolt came right through the the rear of the pistol grip  | |||
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Saeed  A simple trip to the local gun store and then of course the local Rod and Gun club range is all you need to confirm your conclusion I often stand amazed that these guys actually get their moose every year and I would for one love to be a fly on the wall to see how these hunts actually go down ! What is scary is to see the state of their guns and equipment ! | |||
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Again, the purpose of mathematics is a decision and not a number and when making decisions one needs to consider their risk tolerance and people have different risk tolerances. An acceptable risk to one person is an unacceptable risk to another. The accuracy/inaccuracy of the conversion formula or the measurement of brass is relevant to an individuals risk tolerance. To describe the accuracy or inaccuracy of the conversion formula or the measurement of brass as good or bad absent a discussion of risk tolerance is fairly inappropriate. Mike Legistine actu quod scripsi? Never under estimate the internet community's ability to reply to your post with their personal rant about their tangentially related, single occurrence issue. What I have learned on AR, since 2001: 1. The proper answer to: Where is the best place in town to get a steak dinner? is…You should go to Mel's Diner and get the fried chicken. 2. Big game animals can tell the difference between .015 of an inch in diameter, 15 grains of bullet weight, and 150 fps. 3. There is a difference in the performance of two identical projectiles launched at the same velocity if they came from different cartridges. 4. While a double rifle is the perfect DGR, every 375HH bolt gun needs to be modified to carry at least 5 down. 5. While a floor plate and detachable box magazine both use a mechanical latch, only the floor plate latch is reliable. Disregard the fact that every modern military rifle uses a detachable box magazine. 6. The Remington 700 is unreliable regardless of the fact it is the basis of the USMC M40 sniper rifle for 40+ years with no changes to the receiver or extractor and is the choice of more military and law enforcement sniper units than any other rifle. 7. PF actions are not suitable for a DGR and it is irrelevant that the M1, M14, M16, & AK47 which were designed for hunting men that can shoot back are all PF actions. 8. 95 deg F in Africa is different than 95 deg F in TX or CA and that is why you must worry about ammunition temperature in Africa (even though most safaris take place in winter) but not in TX or in CA. 9. The size of a ding in a gun's finish doesn't matter, what matters is whether it’s a safe ding or not. 10. 1 in a row is a trend, 2 in a row is statistically significant, and 3 in a row is an irrefutable fact. 11. Never buy a WSM or RCM cartridge for a safari rifle or your go to rifle in the USA because if they lose your ammo you can't find replacement ammo but don't worry 280 Rem, 338-06, 35 Whelen, and all Weatherby cartridges abound in Africa and back country stores. 12. A well hit animal can run 75 yds. in the open and suddenly drop with no initial blood trail, but the one I shot from 200 yds. away that ran 10 yds. and disappeared into a thicket and was not found was lost because the bullet penciled thru. I am 100% certain of this even though I have no physical evidence. 13. A 300 Win Mag is a 500 yard elk cartridge but a 308 Win is not a 300 yard elk cartridge even though the same bullet is travelling at the same velocity at those respective distances. | |||
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It has nothing to do with math ! The problem at hand is whether we can accurately equate absolute pressure to CUP The crux of the problem lies with the deformation of a slug of copper to effects of pressure. The pressure exerted in our conventional gun is applied as a impulse ie force over time ( more correctly the integral of force over time) The "problem " is that the same amount of deformation can occur under two vastly different impulse curves. We can have deformation when a low pressure ( force) is applied for a very short period of time or a high pressure over a long period of time. | |||
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"accurately equate" equals math. Measuring anything equates to math. You measure something to get a number...and what do you do with the number? at a minimum you compare it to another number and that comparison is a mathematical process. And the purpose of math is to make a decision. All the "pontification" regarding the nature of the measurement problem and the physics is only of value to achieve the decision. In this case, the purpose is making a decision regarding "safety" or lack thereof with respect to pressure. Safety by its nature involves risk tolerances. If the inaccuracy of the measurement or conversion formula or both are within a person's risk tolerance then they can still make the decision; then all of the discussions about impulse curves etc while scientifically interesting (and may feel good) only have value if you need it to help make your decision and if an individual can make a decision without it, it is no longer relevant or needed to achieve the purpose (i.e. the decision). Mike Legistine actu quod scripsi? Never under estimate the internet community's ability to reply to your post with their personal rant about their tangentially related, single occurrence issue. What I have learned on AR, since 2001: 1. The proper answer to: Where is the best place in town to get a steak dinner? is…You should go to Mel's Diner and get the fried chicken. 2. Big game animals can tell the difference between .015 of an inch in diameter, 15 grains of bullet weight, and 150 fps. 3. There is a difference in the performance of two identical projectiles launched at the same velocity if they came from different cartridges. 4. While a double rifle is the perfect DGR, every 375HH bolt gun needs to be modified to carry at least 5 down. 5. While a floor plate and detachable box magazine both use a mechanical latch, only the floor plate latch is reliable. Disregard the fact that every modern military rifle uses a detachable box magazine. 6. The Remington 700 is unreliable regardless of the fact it is the basis of the USMC M40 sniper rifle for 40+ years with no changes to the receiver or extractor and is the choice of more military and law enforcement sniper units than any other rifle. 7. PF actions are not suitable for a DGR and it is irrelevant that the M1, M14, M16, & AK47 which were designed for hunting men that can shoot back are all PF actions. 8. 95 deg F in Africa is different than 95 deg F in TX or CA and that is why you must worry about ammunition temperature in Africa (even though most safaris take place in winter) but not in TX or in CA. 9. The size of a ding in a gun's finish doesn't matter, what matters is whether it’s a safe ding or not. 10. 1 in a row is a trend, 2 in a row is statistically significant, and 3 in a row is an irrefutable fact. 11. Never buy a WSM or RCM cartridge for a safari rifle or your go to rifle in the USA because if they lose your ammo you can't find replacement ammo but don't worry 280 Rem, 338-06, 35 Whelen, and all Weatherby cartridges abound in Africa and back country stores. 12. A well hit animal can run 75 yds. in the open and suddenly drop with no initial blood trail, but the one I shot from 200 yds. away that ran 10 yds. and disappeared into a thicket and was not found was lost because the bullet penciled thru. I am 100% certain of this even though I have no physical evidence. 13. A 300 Win Mag is a 500 yard elk cartridge but a 308 Win is not a 300 yard elk cartridge even though the same bullet is travelling at the same velocity at those respective distances. | |||
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Nicely stated.  roger  Old age is a high price to pay for maturity!!! Some never pay and some pay and never reap the reward. Wisdom comes with age! Sometimes age comes alone.. | |||
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Very interesting. Good to have the knowledge of that formula.I will admit that I am using more current manuals these days but when I started loading in the early 70's my Lyman manual was only listed in CUP.Thanks for sharing the info. Never mistake motion for action. | |||
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