doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure/time curves

Brittany Man asked about pressure/time curves on anther thread
Pressure/time curve from Neil Winston www.claytargettesting.com



If the peak pressure is at .7 msec, and the shot leaves the barrel (?30") at 3.5 msec, the peak pressure would be 5-6" from the breech.

Red Dot appears to peak about .3 msec



Which does not fit with this oft posted pressure/distance curve showing peak pressure within the chamber



Nor this from Sporting Guns and Gunpowders: Comprising a Selection from Reports of Experiments, and Other Articles Published in the "Field" Newspaper, Relative to Firearms and Explosives, 1897



Sherman Bell�s pressure testing published in The Double Gun Journal Summer 2002 "Finding Out for Myself, Part VI, Smokeless vs Black", p.19, and summarized in Volume 17: Issue 4, Winter 2006, p. 39

1 1/4 oz. 3 3/4 dram GOEX FFFg Black Powder at 1240 fps and equivalent load Blue Dot

..............1 inch........6 inches....12 inches
GOEX........5900 psi......4100........2100
Blue Dot.....6000..........4300........2300

The data IS consistant.

The top graph is pressure/time.

The others are pressure/distance.

"If the peak pressure is at .7 msec, and the shot leaves the barrel (?30") at 3.5 msec, the peak pressure would be 5-6" from the breech." - is an incorrect conclusion because the velocity isn't linear. You are accelerating something from a standing start.

The peak pressure is in the chamber, thus "chamber pressure" and it goes down as the expansion progresses.

The area under the curve gives the total energy transfer.

Thanks for posting the graphs. I especially liked the PB / Red Dot comparison as I have shot 7/8 oz 12 ga loads with PB @ 1200 fps for a long time.

I had forgotten about Sherman Bell's data in the DGJ. I'll dig those issues out.

Very well explained, Sgj!! Really on the point with it.

DDA

Thank you gentlemen.
When does movement of the ejecta begin, using the 1 1/8 oz. 1200 fps Gold Medal load?

As Shotgunjones so excellently explained the velocity is not linear but accelerating all the way down the bbl. In fact some significant amount of that initial pressure rise will occur before the shot moves at all just in overcoming its inertia & friction to send it on its journey down the tube. So some of those little bits of milliseconds are consumed in an essentially stationary position.

Considering that the ejecta consists of primer propellant, powder, wad, shot, and trapped air, you can "asume" that there is movement of ejecta upon primer ignition. The primer gas will occupy the powder space almost instantly; a small forward movement of part of the ejecta. The primer gas will have enough pressure to start compression of the wad coussion section; another forward movement of the ejecta. The combined primer and powder gas will quickly have sufficient force (pressure acting over an area) to overcome static friction of the wad which will allow the wad to act on the shot and trapped air. As you can see, movement will start virtually immediately. Significant movement of shot takes only a little bit longer. As the load approaches peak pressure it has something like 4,000 pounds of force acting on less than 1 1/2 oz (like 1/10 pound). And that, good shooters, means some serious acceleration.

Drew, I'm not sure this answers your question. If not, tell me and I'll try some other tack.

DDA

As a point of interest, in the text below the graph comparing the pressure curve of Red Dot to PB the question is asked "who could tell the difference" & I am assuming they are talking about felt recoil.

I was a skeptic for a long time about slow powders producing less felt recoil but a couple of experiences are changing my mind about this.

Note that Red Dot is listed @ 6, Clays @ 7, 700X @ 8 & PB @ 18 on the relative burn rate chart

1. For about 5 years I shot a 12 ga 7/8 oz load using IMR 700X @ 1200 FPS & then wanting to lower pressures I switched to a PB
Load @ 1200 FPS (no other changes, same primer, hull & wad same gun & published pressures only 1100 psi less than the 700X load) & my initial impression was "this kicks less" but I wrote it off to my imagination but have used the PB load as my main target load from that point on.

2.Wanting another 12 ga 7/8 oz load that provided a more open close range pattern for fixed choke guns I started loading the Piston Skeet wad sold by Ballistic Products with Hogdgon Clays @1200 FPS (same primer & hull, same gun & published pressure 2800 PSI greater than the PB load) & my initial impression was " this kicks more than the PB load"

My wife who is an excellent shot but never loaded a shell or read a page in a loading manual in her life was shooting 5 stand with me one day using her checkered butt 6 1/2 lb 12 ga SxS bird gun & the 7/8 oz PB load, ran out of shells & picked up a box of the 7/8 oz Clays load by mistake & after shooting a few asked me "why do these shells kick harder"? True story!

I am starting to think the lower pressure slower pressure curve theory has some merit in reducing felt recoil although in the Clays vs. PB example it's not a good comparison with the different wad column. I've never weighed the wads to see if there is a difference in weight but there is 2.5 grains more powder in the PB load which would add to the total weight of the ejecta & likely offsets any increase in wad weight with the Clays load.

Gough Thomas reports on a "blind" test conducted in England, in which shooters were asked to fire two different loads that were identical in terms of shot charge and velocity, the only difference being the powder. They were unanimous in selecting the load with the faster burning powder as producing less recoil. Going strictly by measurable recoil, that seems logical because although the shot charge and velocity of both loads was the same, you use less fast burning powder than you do slow burning powder. Thus, the total ejecta weight is reduced in the load with faster-burning powder. Not much difference in terms of weight, but perhaps enough for the shooters in question to notice a difference.

There are many factors which affect "Felt" recoil. When firing a load giving a sharp muzzle blast good hearing protection can cause one to "Feel" less recoil. Now study that one, its all in the mind in this case, but one actually thinks they feel a difference. Also keep in mind & never forget if you are just saying two loads have the same velocity based on listed velocity from a load manual until they have been fired over a reliable chronograph you don't Really Know they have equal velocities.
Thomas didn't give us much data to go on on the "Blind Test" he reported on. I very highly suspect, especially as the "Test" was conducted by the Company selling the shells the comparison was made using the old bulk powder against a dense powder. In this case the actual weights of the powder charge could have been very close to a 2:1 ratio, or for a 3 dram equiv load something on the order of 42 grains of powder versus something in the low 20's.