Throat/bore errosion may be an issue if a lot of really light ammo is fired (high velocity.)
Throat, or leade, erosion is caused by overheating the area in question. This overheating comes from the burn temperature of the powder charge
and not letting the bore cool between shots.
There is a direct correlation between pressure and burn temperature. I'm working of the top of my head (as always, it seems) but my memory has it that a 55,000 psi chamber pressure is running in the 3,500 to 4,000 degree F area; just for a microsecond. That is cutting torch temperature, and that is why the leade burns out long before the muzzle is worn.
When Ackley was building the 1-5 twist Wasp, he was researching the effects of twist on pressure. He could not find an effect. Later researchers with more precise equipment found twist could influence pressure up to half a percent.
Twist is not going to effect leade erosion in any measurable sense.
Think of it this way, the *bullet* is still spinning 1 full twist in every 9" of flight, after it leaves the muzzle. At any velocity. (from 100fps to 10,000fps.)
And it is this twist/PER inch that achives stabalization. Velocity basically comes very close to cancling itself out. A few percent of a factor in the 100-10,000fps range at the most.
Correctamundo! Rotational momentum is retained long past lineral velocity. So, at some point (say 100 to 150 yards downrange, just for discussion) the round will settle down and fly true. However, because while it was yawing and wobbling out to 100 yards, it's off course. Simply because it trues up doesn't mean it will magically return to the line of sight.
Greenhill's formula is
Minimum Twist Rate = (K times Bullet Diameter {squared}) divided by Bullet Length
The constant "K" is assigned a value of 150 when the velocities are around 1500 f/s and 180 when velocities get around 2800 f/s.
When Antherd says
...it looks like velocity and the extra RPM cancel each other out. (both are approximately V^2 functions.)
I think this approximated constant is why. The value of "K" must increase in relation to velocity. In English, the bullet has to be spun faster at higher velocity to overcome the increased wind resistance. But I'm not enough of a mathematician to figure the degree of increase. I can see it's not linear.
Greenhill didn't need to do much better. In his time, black powder was the primary propellant, and BP velocities top out at about 1600 to 1700 f/s. Additionally, all his research was done with cannon.
Any thoughts?
