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Here is where I am going with this. With a steel projectile, cut a groove as shown that is a right triangle whose hypotenuse was the depth of the groove plus the depth of the rifling in the rifled cannon barrel. My theory is that upon firing the gas pressure would collapse the notch forcing the hypotenuse, down and out, engaging the rifling.

Idea 1. Machine from steel
Idea 2. Cast from lead. These would be heavier than steel of course
Idea 3. Cast from Zinc. Normally Zinc is not considered malleable. But above 212 degrees it starts become malleable. So , if the projectile is heated to more than 212 degrees by the burning powder gases then the hypotenuse should bend, in theory.
 

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Sounds interesting, Double D. First of all, don't bother with any steel other than 12L14. We tried at least six different types which were the softest and most malleable of them all. 12L14 worked best, by far!! 12L14 is not expensive either, just a wee bit more than 1026 or 1018. 12L14 was the only steel that we tried which NEVER LOST pieces of the rifling engagement skirt upon firing. We agree with your Idea 1. Lead is way too heavy; casting from zinc would assume that the hypotenuse would attain more than 212 degrees F. in just several thousandths of a second that it would be exposed to the heat necessary to change its malleability substantially. Together we say that logic is against you if you make that assumption. Our experience over the past 20 years points to 12L14 steel. It is three times as malleable as 1010 steel which claimed second place.

Mike and Tracy

Now that we have answered your question, please tell us if you can accept our invite described on page 9, posting #175 of the thread called: 2 1/8 Lapan's Foundry Cannon.
 

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Michael Brooks made me some brass 1 inch shells. Is this an option? Is brass soft enough to engage the rifling? Some old gas checks were brass. I can snap a picture if you are interested at seeing them.
 
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