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Member Joined: 28 Oct 2005 Posts: 665 Location: Louisiana

As there is considerable interest in reloads with lower pressures for older guns (as well as some new ones) I thought this information might be of interest.

This information suggests a nominal crimp depth of .050 (1/16" m/l).

Crimp depth-----------PSI
.030 -----------------9,300
.050-----------------10,500
.070 ----------------11,900
.090-----------------13,100

All data was collected on shells using the same components.

Guess this is good info but I have a hardtime understanding how .020 increases pressure by 1200 psi. or .060 which is almost 1/16" causes a pressure rise of 3800 psi. Like I said --good info but I wonder where it came from Smile

I hope that's not accurate. I agree with Billy that it sounds like quite a bit of pressure increase for 1/16" of crimp depth.

Skip

Member Joined: 27 Jun 2005 Posts: 1545 Location: Michigan

I agree, I don't know where that data come from. But I don't believe it. Confused

Woodcock, I could see where this could be a problem becasue you are putting more pressure into a smaller area. I also believe that you would start to see the shell bulge when you went too deep. In using Mec reloaders most wad pressure is just enough to seat the wad.
In the last one of .090 that is almost 3/32", that's pretty deep for a crimp.

I'm guessing that you had these shells with the different depth crimps sent for pressures testing?

Also I wouldn't consider even the first one as being low-pressure.

Think fire cracker in open hand vs fire cracker in closed hand. Same charge, significantly different result.

Member Joined: 27 Jun 2005 Posts: 1545 Location: Michigan

Member Joined: 28 Oct 2005 Posts: 665 Location: Louisiana

I am not in the habit of posting unsubstantiated information on this site.
You may verify these figures from Hodgdon. They were derived from work conducted by Hodgon [/i]ballisticians using "12 guage shotshells loaded in the same type hull with the same amount of propellant, the same primer, wad and shot charge, ONLY THE CRIMP DEPTH WAS CHANGED."
Ron

I can see how that would happen. Works the same way with water, more crimp, higher pressure. A fire hose nozzle automatically adjusts to the volume to keep the pressure up. Just a little physics from college.

Member Joined: 28 Oct 2005 Posts: 665 Location: Louisiana

Same thing happens when either constant (pressure/volume) changes--a variation of Boyle's Law I believe.
In the case of crimp depth, I suspect that the resistance of the crimp to unfolding is of primary significance as the volume change is minor. However, it is just that sort of 'intuitive' thinking about expolsives that can leave one without a full compliment of fingers.
Keep in mind that a recipe that calls for a 6-point crimp has a different pressure when roll crimped.

Member Joined: 28 Oct 2005 Posts: 665 Location: Louisiana

Joined: 06 Apr 2007 Posts: 70

what's awkward to grasp is that the pressure buildup is almost instantaneous and the crimp can't be blown open fast enough to relieve it.

Hello Woodcock :

You bring logic and learning to the forum.

One thing extra is that the deeper crimp captivates the leading edge of the wad and holds it in place longer and more decisivly thus the pressures mount in a hurry compaired to a load with an over the shot card which allows the crimp to open up at a uniform rate versus a shell using just a crimp alone.

The old Blue Magics used to shoot the ends of the hull off or the whole hull for that matter would go out the end of the bbl if you put a deep crimp on that hull.

I must agree with the testing and data that BPI came up a long time ago concerning this subject just use an over the shot card and see how much nicer your crimps look as well as how consistant the load will be , and I recall the reps putting out the data on the crimps at the trapshoots in the past .

Regards Charles