Polygonal Rifling

[nkbj]

Getting ready to order another barrel with .458" and .470" groove but this time 33" long with 20" twist. Thinking about a heptagonal cross section with roundly blunted vertices to minimize the amount of bullet expansion needed to seal the bore. So I put pencil to paper to figger the difference between the radius of what would be the .458" bore diameter and the vertices of the "grooves" for an ideal regular seven sided polygon versus limiting the groove diameter to .470". What I came up with was that from center point to the bore wall would be 0.229" (half of .458") and to the corners (vertices) would be 0.254"...
That is effectively a 25/1000ths "groove depth" and way much more than what you'd expect in "round ball rifling". So, holy mole', if I loaded with .457" diameter bullets in a perfect heptagonal bore they would exit the muzzle at dang near .51 caliber! I'd just never looked at it that way before.

And then I started thinking, hey, but wait a minute, what about people that shoot round bullets in their .451 caliber Whitworth rifles with hexagonal bores? How much expanding is going on in there?

So plugging in the numbers I came up with even larger amounts of expansion being needed due to the more sharply angled vertices in the hexagon. So this morning I got a new way of looking at muzzle loading rifles. And I'll stick with the idea of having bluntly rounded vertices on the hexagon.

 

[Loyalist Dave]

And then I started thinking, hey, but wait a minute, what about people that shoot round bullets in their .451 caliber Whitworth rifles with hexagonal bores? How much expanding is going on in there?

It's been pretty well established that patched round ball does not obturate larger in the bore when fired. It has to do with the round surface of the sphere where the expanding gasses apply pressure.

LD