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The riflings were introduced(first in small arms) because the non-rifled barrels offer no mean of stabilizing the shell. Therefore the shell(or bullet) would be deviated by the friction with the air, up to the point that it will hit anything but the target.
(The 18th century musket soldiers fought in a battle line, shoulder to shoulder, because the inaccuracy of the smoothbore muskets they used compelled them to concentrate their fire to increase the chances of hitting more enemies)
By rifling the barrel, the projectile is stabilized, because it turns around its longitudinal axis. By "revolving", the flying projectile "pierces" the atmospheric layers, having a far straighter path and far better accuracy.
But a rifled barrel offers much less muzzle velocity at the same powder charge. Therefore, to attain the same muzzle velocity, the powder charge has to be increased.
The thickest armor of a 1945 tank was that of the PzKpfw VI B "King Tiger", about 180mm(Soviet IS-3, with 220-230mm doesn't count, it did not take part in WWII battles). And even this armor could be easily penetrated by a hand-held Panzerfaust. Rifled barrels offered enough muzzle velocity to break this armor, and so the rifling was kept to maintain accuracy.
Nowadays tanks have far thicker armor. The armor on the Leopard 2 is classified, but it was estimated to be about 400-450mm, thicker than the belt of a battleship(!!!) And it's no longer cast & bolted or rolled & bolted steel, but multi-layered composite material. To break this kind of armor are needed super-high-velocity subcalibre shells, the famous APFSDS. To launch these shells with a sabot from a rifled barrel, would be needed huge quantities of powder, and the barrel would go unusable after 40-50 rounds or less, like the barrel of the WWI Paris Gun did after 65 rounds. Therefore, there are used smoothbore barrels, to give high muzzle velocity with low powder charge, and to resist wear.
To stabilize the shell are used both tailfins, and the electronic control of the gun, via a chip that takes into consideration the range, charge, type of ammo, but also atmospheric conditions.
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More of the modern tanks are going with a smoothbore cannon instead of the rifled barrel. Is this due to the different shaped charges used as ammunition, or do the higher velocity rounds work better in smoothbore? I believe that most WWII era tanks used rifled barrels, now the Leopard and the Abrams use a smoothbore gun. The Challanger however stills uses the rifled barrel. Anyone have information about these two types, and which is better?
Today, the smoothbore tends to be the better choice, but you need to understand ballastic for a explanation to make sense. I don't know how much you now, so I'll be exceedingly simple, I'm not trying to be insulting.
A rifled barrel refers to the fact that the barrel has a rifle, or small groves cut into the inside of the barrel. These groves twist around the barrel in a sprial. You'll sometimes see this refered to as a 1.5 twist (or some other number. This means the rifle groove makes 1.5 complete turns inside the barrel.
When a tight fitting round is fired through a rifled barrel, the grooves force the round to spin. This spin helps stablilize the round. Thus the round travels further and straighter than it otherwise would.
Because the rifling cuts into the round, it creates resistance that must be overcome for the round to travel. This means it takes more power to propel the round. Plus, the rifling is subject to wear. As the rifling wears, the advantages gained by the rifling decrease. This is why rifle barrels on tanks are often made in multiple parts, because the wear patterns are different.
A smoothbore doesn't have any of these problems. There is no rifle resistance to overcome, and no rifle grooves to wear out. What a smoothbore doesn't do, is impart spin on the round. Thus a smoothbore and a rifled bore firing the same round, the smoothbore would have less distance and accuracy, though the smoothbore round would be going faster.
Modern technology allows smoothbores to fire with the same accuracy as a rifled bore. This is because the rounds themselves are modified. Nearly every smoothbore anti-tank round is fin stabilzed. Small fins are released right after the round leaves the barrel. These fins provide the same stability as the rifling, without the disadvantages.
Plus, smoothbore allow you to fire rounds that cannot be fired with a rifled barrel. Think of a giant shotgun firing many pellets. You can safely fire these out of a smoothbore, while it would destroy any rifling if it were in the barrel.
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The BIG problem was that the efficiency of shaped charge rounds is seriously degraded if they are spinning.
Now we have to take a big step. Shaped charge rounds used to have a secondary functionality as 'bunker busters' and so on. I think that ensured that they are still around today even though modern US tank ammunition has been designed to be useful only against other tanks. Hopefully no one will take advantage of this 'tunnel vision' in US millitary thinking. (The Abrams is no more useful in close in-combat than the Elefants were at Kursk.)
Over the years we all came up with 'wonder-warheads' that would do this and that, but in order to keep the HEAT round effective, we had to stop the spin. So we came up with rounds that used teflon rotating bands to grip the rifling, but there were roller bearings inside the bands to try to keep the round from spinning. It kind of worked because the projectiles didn't spin as much, but it surely irritated the infantrymen in the areas where the rotating bands and roller bearings landed.
Anyway, the US hasn't produced a tank gun designed in the US since before we went to the 105mm. The smooth-bore guns are nice but not practical for what tanks were originally designed to do. The 120mm smooth-bore is an anti-tank weapon, but there are a lot of better anti-tank weapons available. The Cavalry is riding again. It is quite possible to hit and kill an individual infantryman with an Abram's main gun, but is that a worthwhile goal? Especially when the Abrams can't kill infantrymen in groups at less than 1200 meters without using a radio?
Smoothbore has its place, even in tanks, but not all tanks. Some need to be able to support the PBI. I really don't think that smoothbore guntubes last longer than rifled ones, especially considering the amount of powder we stuff into them now.
Finally, and I know you have been waiting for this, what about being able to see the target? North Africa is an exception. During two years in Germany I was never able to range to a target more than 3500 meters away due to moisture in the air, which also effects lasers. So what good does it do to have a weapon that can kill a tank at 5000 meters, but nothing else?