doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Tom Roster SSM article re low vs high pressure

Glad my memory isn't failing. Seems we agree that there just isn't a lot of point to working up loads that are much lower pressure than that as far as safety is concerned.

But I remain interested in determining just how much advantage a high pressure load offers vs low pressure in terms of retained pressure at various points further down the barrel. Thought I recalled that Sherman Bell gave some examples of that in one of his "Finding Out For Myself" articles in Double Gun Journal, and found it in my files. It's Part VI, "Smokeless vs. Black", Summer 2002. In that article, Bell's goal was to repeat what we see in the Dupont tests from the 30's, comparing smokeless to black--but using modern smokeless powders. But in running those tests, he provides some data on retained pressure down the barrel. He placed strain gauges at 1, 3, 6, 9, and 12 inches. Fired 10 shots with each load he tested to get an average. All of his loads were low pressure--nothing over 7,000 psi--but there's enough difference between the lowest and highest pressure loads he tested to make an interesting comparison.

He tested two comparable loads, one using 7625 at a one inch pressure of 4800 psi; the other a black powder load at 4700 psi. At 12 inches, the pressure from both was 1800 psi. He also compared a load using Universal Clays that read 6900 psi at one inch, along with a black powder load at 5900 psi. 12 inch pressure of the Clays load was 2000 psi; the black powder load, 2100 psi.

While the two higher pressure loads shed more pressure in 12" than did the two lower pressure loads, they still retained slightly higher pressure at 12". So we're not seeing any gain yet, at that point. It's too bad he didn't use any higher pressure loads. Undoubtedly a 10,000 psi load would drop in pressure even more over 12" than any of those lower pressure loads. But it also has a lot further to drop before it catches up with them.

So at what point down the barrel does the higher pressure load drop below the pressure of the lower pressure load? The graph from the Dupont test shows that the loads with the highest and the lowest peak pressures have flip-flopped at 3", with the lowest becoming the highest, and the highest dropping to the lowest. That doesn't happen in Bell's test. The two low pressure loads (black and smokeless, respectively) have dropped to 3700 and 3800 psi at 3", while the two higher pressure loads (again, black and smokeless respectively) are at 4700 and 5100. Low pressure loads are 2400 and 2300 at 9"; high pressure, 2900 and 2700. But the loads in Bell's test are not "twins" to the same degree they were in the Dupont test, where they were all 3 dram equivalent, 1 1/4 oz shot--which likely makes a difference. Likewise, Bell's use of modern smokeless powders may also be a factor.

It would be interesting to run the same test with all modern smokeless loads, same shot charge, same velocity, but with widely different peak pressures. Would the results look more like the Dupont graph, or more like Bell's results? It would certainly be a good subject for yet another "finding out for myself" series of tests.

"Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics" by Wallace H Coxe, 1927
"All powders loaded to develop the same energy" (ie. the area underneath each curve)



Pressure is expressed in Long Tons. sq/ inch but it seems clear that PSI X 2240 was used to calculate those values (+ 10-14% for modern transducer numbers)

Ballistite maximum pressure at 1� was 4.9 Long Tons = 10,976 psi
Schultze at 1 3/4� was 4.5 Long Tons = 10,080 psi
DuPont Bulk at 1 2/3� was 4.1 Long Tons = 9,184 psi
FFFg at 1 1/2� was 3.8 Long Tons = 8,512 psi
DuPont Oval at 2� was 3.5 Long Tons = 7,840 psi


Pressure-time curve comparing Red Dot and PB, from http://www.claytargettesting.com/study2/Study2.3.pdf




Alliant data for 1 1/8 @ 1200fps 12g
Alliant e3 - 17.2 grains = 10,950 psi
Red Dot - 18.3 gr = 10,300 psi
Green Dot - 21 gr = 8900 psi
Unique - 22 grains = 8900 psi
Green Dot and Unique pressure curves are indistinguishable.
See http://www.shotgunworld.com/bbs/viewtopic.php?f=13&t=405370



Pressure-distance curves are almost identical at about 2"

Larry;
Good questions all, you are starting to sound like you may actually be interested in learning a bit. There are in my opinion a couple of key points involved here. One you have already mentioned is that DuPont's tests were all made with loads having the same terminal ballistics, but with widely differing burn rates of powders.

Bell's tests on the other hand, as you also mentioned were made with loads of differing terminal ballistics & with a far lesser variation of burning speeds. I looked up Universal Clays on Hodgdon's burn rate chart & only found Universal without the Clays attached. I do not know if this is exactly the same powder or not.

The burn rate for Universal falls between Unique & Herco. in comparison to Alliant powders so is just a bit faster than 7625. Black powder burn rate is close to the 7625 so it was, in fact, a slow burn powder. The early smokeless powders were all faster than black so gave higher max chamber pressures than did Black, but lower barrel pressures. I believe that DuPont Oval was the first of the so-called progressive powders. When Hercules brought out Herco it was "I Think" just a tad slower than Oval.

As you can see thus Bell's test were all done with powders of very close to the same burning rates but with different terminal ballistics. so, unfortunately, do not provide much help as far as this discussion goes.

Incidentally, the Infallible powder used in the DuPont tests had the exact same chemical composition as Unique. Both were flake powders, but the Unique flakes were about 16% thicker than the Infallible flakes so just a WEE bit slower burning. In these DuPont tests, Black fell between Infallible & Oval. Ballistite & DuPont Bulk Shotgun smokeless was just a shade faster than Infallible but were used in similar loads. Oval was designed specifically for the new Higher Velocity loads with the maximum shot weight, which for 2 3/4" 12 gauge was 1� oz. The early 3" 12 gauge Magnum carried 1 3/8 oz & the so-called Super 10 (2 7/8") 1 5/8 oz. DuPont specifically stated the 3-dram equivalent load of Oval, even with the full amount of 1� oz shot was an "Underload".

My loading has always been aimed toward hunting loads so a powder which works well at any temperature encountered is desirable. With this in mind, I believe one will have far better results to stick with powders designed for the purpose. For light shot charges thus a powder in the fast range is desirable. For heavy charges at maximum velocity then a powder in the slow range is called for. Medium powders are for the intermediate ranges.

For strictly warm to hot weather conditions, a bit more leeway is available, but I still see no need what-so-ever to go to those extreme low pressures by taking a slow burn rate powder out of its design element, even though some loading manuals list such loads.

I will note that unless it has occurred in the very late years Hercules/Alliant has not listed any extreme low-pressure loads with their slow powders, For Good Reason, I believe.

Did you have access to the Sherman B. report yesterday, with different potential peak pressures and pressure cross over points as it came down?

Our discussion from 2012
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=291214&page=1

Larry: what was the load (shot and Dr.Eq. or fps) of Bell's 7625 and BP, and Universal Clays and BP please?

Drew, first 2 loads were 1 1/8 oz, 3 DE. The 2nd pair is basically the classic Super-X load: 3 3/4 DE, 1 1/4 oz.

Miller, the Universal you found is the powder Bell used. Back then, usually called Universal Clays. Thanks for your thoughts. Would be interesting to do tests with a greater pressure spread using different modern smokeless loads.

Thanks Larry.

titewad "killer bees" are the softest shooting shells I have ever found. Been very informative following your posts, though, 2 piper. I don't have an explanation for why I have experienced such wicked recoil with Longshot, and I don't mind shooting my 375 or seven mag. In fact I have never considered my 30'06 to recoil substantially even with the 220 grainers I took elk hunting once. Longshot just seems to have the right wave length to ring my bell and make me feel "head-achy." I wonder if vibration patterns work in both directions?

Is there a difference in muzzle rise between fast and slow powders? Could a slow powder actually put the comb of the stock into your cheekbone a bit more. The results I have had have been with several guns, so stock fit may not be the answer.

I haven't used Longshot since 20/28 came on the market. A lot of shooters will remark that it's loud, and that may have a connection to the felt recoil. Somewhere along the line, Bob Brister wrote that the best recoil reducer is good hearing protection. Sounds louder, you expect it to kick more.