Max load for 158g swc and unique?

[elano]

I've been shooting 6.5g under a 158g missouri boolit, the 357 mag 18bh ones. They feel a little on the soft side to me. Is it safe to bump up the charge from 6.5 maybe to 7.5 without risking overpressure? I know some older manuals list a max charge as high as 8 grains while some newer ones only go up to around 6.8 for cast boolits. However the same manual may lists over 8 grains for a jacketed bullet. I like the way 7.5 grains feels with my xtps. Just curious what some opinions are.

[Larry Gibson]

NO

Happens I just pressure tested 150 gr 358477 over 7.5 gr of Unique in R-P nickled cases with WSP primers yesterday. Average psi(M43) for the 10 shot string was 37,300 with 3 of the shots ranging up into the 38,100 - 39,700 psi range. SAAMI MAP is 35,000 psi for the .357 mag. That was out of my 8" Contender test barrel. Accuracy also sucked BTW.

I consider 6.8 gr Unique to be a max load with my 358156 GC'd 158 gr bullet.

Larry Gibson

[Treeman]

With due deference to Mr. Gibson and his test results I must point out that 7.5 of Unique and 158 gr lead bullets has been tested at 28,000 CUP. Given the numerous manuals that listed such loads and in light of the strange gelding of the .357 mag with the change to PSI Piezo or strain gauge versus 42000 CUP I'd shoot 7.5 with 150s.................but I'd rather choose a different powder.

[elano]

Interesting. So then why would the manual recommend such a stouter load as safe for jacketed?

[Larry Gibson]

With due deferenc to Treeman and his manual search I must point out that comparing psi (piezo) to CUP is like comparing apples to oranges.

Lyman has not listed Unique with any bullets heavier tha 141 gr in the 3rd and 4th editions of their Cast Bullet Handbook. The max load with the 141 gr bullet listed in the 3rd Edition is 7.9 gr at 40,300 CUP. That is a top end load. That load is not listed in the 4th Edition (hmmmm, I wonder why????). So should we wonder that 7.5 gr Unique under the heavier 150 gr bullet is 2,300 psi over the SAAMI Max? And then what would it be under the even heavier 158 gr bullet the OP asks about????

There is lot's of older data measured by CHP or CUP that has been found to be over pressure using modern piezo measurements. That's why a lot of data, especially in straight sided cases, has been reduced. My answer is still NO.

Larry Gibson

[101VooDoo]

elano,

If you want more oomph in your load, no point in pushing unique to the max. Just pick another (slower) powder.

[elano]

I decided to go for 7.0 grains. It seemed to have a little more power than 6.5g, but some of the shells didn't extract as easily as the 14g of 2400 I tried next, nor did they feel as powerful. This leads me to wonder, why does 7.5g work so well with jacketed bullets but not with lead. Hmmmm...

[357shooter]

For the same reason jacketed bullets produce more muzzle flash, compared to a good fitting boolit. The jacketed doesn't seal nearly as good and the expanding gas blows right by.

[zxcvbob]

Try 8.0 grains of Herco or 11.0 grains of AA#7. (10.5 grains of Blue Dot should be a good load, but I got excessive leading with it) These are max loads, so work up to them. I might push Unique to 6.8 grains but I don't think I'd take it to 7. It's probably not dangerous, just not likely to be a good load.

[Lonegun1894]

I use 7.0 grs Unique under a Lee 158 both PB and w/ GC, and they work well in a Ruger Security Six 4" and a Rossi '92 20". The Security Six gives 4" groups at 50yds, and has taken a deer with this load. The Rossi gives 3-4" groups at 100yds, and 8-10 at 200 yds with this load also, but I have not had a chance to test it on game yet. Neither gun gives any leading and this is the most accurate Unique load I have found for either of these guns to date. I did go higher without any pressure signs in these two guns, but this was the most accurate load so I stuck with it.

Having said that, I tried working up to this load with a Taurus 605 I have, and started getting sticky extraction at 6.7 grs along with heavy leading, so never did get to the 7.0 I was hoping to be able to use for all three guns. This is just to say that not all guns are equal or handle the same loads equally, as my testing showed with mine. I also do NOT have access to pressure testing equipment, so I can not tell you what kind of pressures these loads develop, other than saying they are safe in 2 of my 3 guns. Just be careful and stay safe.

[olafhardt]

I am a retired engineer. I spent a lot of time in measurment labs. I know this may open a can of worms, but when I read of measuring chamber pressure, I shake my head. Piezo's, strain gauges, crushers give at best an estimate of pressure, a SWAG if you will. This means start low, go slow and stay below max. BTW I don't gripe about this because I can't think of anything better!!

[Larry Gibson]

olafhardt

That is a perplexing problem. They haven't come up with a way of putting something inside the case to measure the psi. Even then the volume of what was inside would require an offset. Piezo transducers and strain gauges give us the best method so far devised. Yes there are offsets and "constants" that are added into the computations but then most scientific calculations use those too.

As to my measurements I have "calibrated" the M43 with various cartridges using "reference ammuntion" of know psi (at least "known" according to the manufacurer). Also I have used 6 different "reference" ammuntions of standard and +P loadings to use for comparative purposes (some erroneously refer to this as "calibrating") in the test barrel used for the above posted data. One may indeed question it but as you say "I can't think of anything better!!"

BTW; my older manuals lists 6.8 gr as max for Unique with 150 - 158 gr bullets. That pretty much matches what the data from my one test of 7.5 gr with the 150 gr bullet suggests.

Larry Gibson

[chboats]

Larry Glibson - I have a question. What effect on pressure does the cylinder gap on a revolver have? Your tests were done with a contender.
I have never had a need to push the max. Eerything lasts longer that, the gun and brass.
Carl

[Larry Gibson]

Larry Glibson - I have a question. What effect on pressure does the cylinder gap on a revolver have? Your tests were done with a contender.
I have never had a need to push the max. Eerything lasts longer that, the gun and brass.
Carl

Psi will be less because the cylinder throats are essntially freebore that lengthens the chamber volume and because the barrel/cylinder gap will vent some of the spi. However, with many of the faster burning powders such as Unique the peak psi may be reached before the bullet base clears the barrel cylinder gap. Depends on length of bullet bearing surface and length of cylinder.

Larry Gibson

[olafhardt]

Larry, I truly agree with what you say; however to truly calibrate an instrument requires a primary standard. I am not aware of one for these pressures. Also critical measurementts on a dynamic system require the system reach steady state which a gun doesn't do during firing. Bulges and barrel failure and indeed some electrical measurements indicate that max pressure does not necessarriarly occur in the chamber. I cannot find sufficient reason to beleave that the combustion reactions are compleated in the barrel. Accurate measurements require more than hard work and persistance. You don't get as much out of it as you put into it. Still this is the best we have.

[Nueces]

Years ago, pre-www, Bill Caldwell was an engineer at Speer (I think). He instrumented a S&W M-28 by making a one chamber cylinder that incorporated a transducer, so he could investigate cylinder gap effects on revolver chamber pressures.

His results often showed higher pressures in the revolver in comparison to a rifle type chamber. He postulated that the bullets were slowed during forcing cone passage and that the pressure rose because of that.

He was killed soon after beginning this research and no one continued his work.

I'm sure I have the original article describing this, likely in an old Handloader, but I haven't read it in years and may have details wrong. But I'm sure about the increased chamber pressure result in revolvers, as that was the big point he made at the time. His results may have been upended since then because I've heard little to nothing about this issue for a long time.

Mark

[Larry Gibson]

olafhardt

Notice in my post calibrate was in parenthesis because that word is not quite correct. In the world of ballistics where psi is measured with piezo transducers and strain gauges the strain gauge is calibrated at manufacture and that data is inputed into the program. Trans ducers are also calibrated (very expensive) before use in pressure test fixtures. However, both systems are then shot with 'reference ammuntion which is a particular lot of ammuntion "known" for it consistency. This ammuntion is available from most manufacurers and is also very expensive. If interested you can visit SAAMI's site and they list the reference ammuntion available from whom.

This is done because (using .308W for example) Winchesters test barrel is different from Remingtons, which is different from federals, which is different from Black Hills, etc. All those test fixtures with test barrels and calibrated piezo transducers will give different psi readings with the same .308W reference ammuntion for the same reasons difference rifles will give different velocity readings with the same ammuntion, i.e. there are other variables than just the calibration of the transducers or strain gauges which are also used by the major manufactures to test ammuntion in commercial firearms.

They use the reference ammuntion to get an "offset" figure to then add to their test loads. Let's say the .308W reference ammunition has a psi rating of 59,800 psi and when Remington fires it in their test barrel in gives a reading of 57.400 psi. Federal fires it in their test barrel and get 61,200 psi. Remington then uses an offset figure of + 1,400 psi which is added to the psi measurements of their developed .308W ammuntion to get the "offical", if you will, psi measurement. Federal would subtract an off set figure of 1,400 psi to tget their "official" psi figure. That's how it works.

Added explanation; the use of the offset gives a corrected figure for what would be the expected psi of the reference ammunition. As noted in a later post a % using that offset would then be used in actual load development. However, if the developed loads MAP comes with the ES of the reference ammunition then the load variation is within expected normal variation and is usually then accepted at face value.

The altenative to the expense is to get several lots of ammuntion and check with the technicians at the manufacure for the actual psi of those lots when tested. I've found the technicians when they know what i'm doing are very helpful and i thank them for the data they provide. In the case of the M43/test barrel I use and the test of the .357 Magnum loads I also tested that day a specific lot of Winchester 357 Magnum I use as "reference ammuntion". The Winchester "reference ammuntion was measured with in 200 psi of the psi that Winchester recorded. The psi was well within the normal test to test variation. The .38 SPL "reference ammuntion" also tested within normal variation in the same test barrel. Thus we can expect the psi data of the 7.5 gr load of Unique to be valid. Yes, it is still the best system we have.

BTW; the combustion reactions of many powders in smaller capcity stright walled cases fired in shorter handgun length barrels is not completed within the barrel. This is easily seen in the many times erratic pressure traces of the burn in the barrel and on muzzle exit.

Larry Gibson

[williamwaco]

==========================================
I decided to go for 7.0 grains. It seemed to have a little more power than 6.5g, but some of the shells didn't extract as easily as the 14g of 2400 I tried next, nor did they feel as powerful. This leads me to wonder, why does 7.5g work so well with jacketed bullets but not with lead. Hmmmm...
==========================================

This is a very interesting thread and I enjoyed reading it so much I gave it a 5 star rating.
BUT did anyone note the comment from elano about extraction? 14 grains of 2400 is a HOT load in my opinion ( granted some people go higher ) Any load that produces extraction "not as easily" as 14 grains of 2400 is too hot.

elano, Check you manuals, you will note that even with cast bullets of the same weight, different bullet designs will produce different results with the same powder charge.

Please, do not exceed that 6.5 grain charge. If you want something hotter use your 2400.

[Larry Gibson]

Nueces

Actually several reports have been published using strain gauges on revolvers. Due to the shape of the cylinder the psi readings are somewhat lower than in closed breach cylindrical test barrels, even those that are "vented" as used in test fixtures. Octagon barrels also will give lower psi readings.

With medium and slower burning powders the pressure trace does give evidence of when the bullet enters the throat and is sized down or become fully engraved. This can be seen with many cast bullet loads and with some jacketed loads. To the contrary I have seen little evidence of a pressure rise and then let off with .38 SPLs or .44 SPLs when fired in magnum chambers with the longer jump to the throat. I would expect the same increase in psi, if there is really one, to be evident in that situation also. One of these days I may put a strain gauge on a revolver cylinder. Even though the psi data may be low that psi increase when the bullet enters the forcing cone should be evident if it happens..........maybe some day.

Larry Gibson

[wiljen]

The issue I see with what you wrote to olafhardt is that calibration at a single point assumes a linear function and that is rarely true with strain gauges I have used. On mine, if I were to use reference ammo and find it to be 1900 PSI off at 60,000 I would be safer finding the percentage it is off and using that calculation to offset my data rather than just subtracting 1900 PSI at all levels. I would feel much better calculating the offset for my 43b by using samples at 4 different known pressures to establish the curve rather than any single point value.