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Old 07-15-2019, 07:54 AM
Unclenick Unclenick is offline
 
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This may be drifting a bit OT, but here are the basic factors involved:

Spherical grains burn inward from the outside, shrinking their surface area as they go. To be progressive burning, any powder has to make gas at an accelerating rate during burning, so the challenge with spherical powder is to get that burn acceleration despite the surface area simultaneously shrinking. This is done by imposing a deterrent gradient in the grain that is very concentrated on the outside and thinning out exponentially toward the center. That forces the powder to burn very slowly on the outside, but speeding up more quickly than the surface area is shrinking as it burns inward. That heavy deterrent concentration on the outside is what makes spherical powders harder to ignite than their similar burn rate stick counterparts.

Stick grains have heavy deterrent coatings on the outside but not inside their perforations. So the powder inside the perforations lights up easily. The perforations then burn from the inside toward the outside, growing larger in diameter and surface area, accelerating the generation of gas without changing the core burn rate. That means the surface area grows and that provides accelerating gas production at a rate determined by the size and number of perforations and the amount of blended deterrent throughout the grain.

This leaves the stick powder open to playing with deterrent concentrations that affect the ratio of perforation burning from to inside out to the outside burning inward and make it temperature insensitive. I do not know the details of how this is accomplished so I can only speculate that if you applied a light deterrent gradient from the outside of the stick grain before applying the heavy outer deterrent coating (often done by tumbling rather than by solvent penetration, as with spherical), you could modify the progressivity so that it lit quickly but was a little less progressive burning as the perforations widened, which happens at the time the temperature and pressure that speed up powder burning are growing rapidly toward the peak pressure value. In that way, the powder would seem to respond less to changes in pressure and temperature which would mean it acts overall less sensitive to temperature. But the balance of pressure and temperature and volume have to be right for that to compensate.

You can see some evidence that something along those lines is happening when you look at the number of feet per second produced per grain of powder in the recommended load range as I mentioned in my last post. In the .308 and .30-06, Varget always has a slightly lower number than 4064 (and 4064 has a lower number than 4895 and several other powders; a reason it is a favorite target accuracy powder). But in .223, at a couple of different bullet weights, 4064 does better than Varget, indicating the advantage to the Varget formulation has essentially gone away.

BTW, running those numbers for any powder lets you identify insensitivity not only to temperature and pressure but to charge weight errors. That means, if you have a powder measure that errs by a tenth or two from throw to throw, you can expect the powder with the lowest number of fps/grn to show the least performance variation from shot to shot as a result of that.
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