On Sun, Jun 28, 2015 at 11:39 PM, Richard Aiken <raikenclw@gmail.com> wrote:

On Sun, Jun 28, 2015 at 8:14 AM, Grimmund <grimmund@gmail.com> wrote:
Why would you need the sabot to drop away? I mean, in a vacuum,
there's no atmospheric drag due to the increased cross section of the
projectile....

The advantage of the squeezed/tapered bore effect (which is effectively duplicated through the use of APDS ammo in modern cannon) is the increase in muzzle velocity and consequent increased range/penetration.

I'm frankly not sure how a sabot dropping away duplicates the effects of squeezing the caliber of the fired round down by ~30%. I *suspect* it's because the sabot fills out the larger caliber but is substantially lighter, so the acceleration provided by the gasses of the bursting charge is effectively increased in the final fraction of a second of firing, as the sabot falls away just outside the muzzle.

As Grimmund says, the point is lowered air resistance after leaving the barrel. Obviously, you're also concentrating the impact on a much smaller area. Oh, acceleration is increased along the length of the entire barrel--not force, but acceleration

It's exactly the same effect. The tapered bore gun actually has a non-discarding sabot. They look like "skirts" that get squished flat against the sides of the shell. http://weaponsman.com/wp-content/uploads/2015/03/CHAPTER-6-E-FIGURE6E1-PAGE-95.jpg A problem in atmosphere is that the skirts don't always get pressed down perfectly smoothly, which can increase drag and result in some instability.

The squeezed/tapered bore effect would seem to be potentially much the more efficient of the two methods, since you've got the full length of the barrel to use for the purpose of increasing the effective gas pressure.

Both methods use the full length of the barrel. A tapered bore is much less effective because it wastes a lot of energy in additional friction as it squeezes down the skirts.Also, you have to carry the weight of the skirts the whole way to the target, which is a dead loss.

The one advantage of tapered bore is that the volume of the barrel expands more slowly, so you can keep the gas pressure in the barrel a bit more even without playing all sorts of games with the propellant grain.

In the context of a future setting with advanced manufacturing techniques - such as 3D deposition of artifical diamond rather than old-style bore drilling - I suspect that squeezed/tapered bore cannon might make a comeback.

Unlikely. Effective DS killed tapered bores because it was so much more efficient.