I was reading about the M1A1 Abrams and they mentioned they upgraded its main armament from a rifled bore gun to a smooth bore gun.
While I am sure the designers know what they are about this puzzles me. I thought rifled guns were an advancement in weapon technology and viewed smooth bore guns as a thing of the distant past. It was my understanding that a rifled bore, by imparting spin to the projectile, greatly enhanced accuracy.
So how is it that a smooth bore gun is an upgrade for a main battle tank? If a smooth bore actually does have advantages are there smooth bore rifles being made today? What about smooth bore naval cannons?
Tankl cannons are mainly anti-tank weapons, i.e. those using armor piercing shells. The most effective armor piercing round seems to be long and thin. Rifling works best on relatively short, fat rounds. In order to stabilize long, thin rounds the rotation rate would need to be more than rifling can provide. So tank cannons are smooth bore. The actual round is mounted on a sabot that fits the bore and falls away because of high drag after leaving the barrel. The round is stabilized by fins.
From what I’ve read, the smooth-bore gun was selected for optimum performance with APFSDS (armor-piercing fin-stabilized discarding sabot) ammunition. You don’t want a rifled barrel since spinning the projectile/sabot just causes problems. The projectile looks like a giant dart.
Both KE (kinetic energy) and HEAT rounds (also a form of KE round) perform best without spin. The KE long rod penetrators, if spun, would have the forward portion of the projectile under-going tremendous deceleration of the spin while the rear portion would still be trying to spin. Basically the projectile would try to twist apart limiting the length of the rod used before breakup. Not a physics major, but angular momentum is a waste of energy trying to penetrate armor, concrete, etc… It takes more energy to penetrate and twist as opposed to just penetrating.
The Challenger II main battle tank does have a rifled bore to accurately fire one type of projectile, HESH (high explosive squash head), at distance. The KE projectile, CHARM 3, uses a slipping sabot and slipping obturator (gas check) to minimize spin of the penetrator dart going down range. This slipping was also used in other projectiles fired from the older model rifled barreled tanks of different nations.
Some AT projectiles, rocket, missiles, do have a slow rate of spin to augment fin stabilization.
HEAT warheads, if spun during flight, may have a cone/liner which is twisted in the opposite direction to minimize breakup of the penetrating jet.
Or you can fire a HEAT or AP (or Armor-piercing, discarding sabot, APDS) shell fitted with a band than spins, while fins stabilize the shell. THen you get an APDSFS round which is scary to even spell.
Also smooth bore tank guns can get much higher muzzle velocities (ciritcal for KE penetrators), suffer far less from barrel wear, and are lighter (I think they are also shorter). All in all, they rawk, which is why all modern MBTs seem to be moving to them (despite them being introduced by the soviets in the 70s to cries of derision).
For small caliber weapons and conventional artillery, not so much, since for both it’s simpler and more convenient to stick with the ‘stumpy projectile spinning on its axis’ approach.
In simplistic terms, you need something to stabilize the projectile while it’s in flight. With tank guns, the round is big enough (and in the case of APFSDS, the correct shape) that you can put fins on it to achieve the same effect as rifling. Way back there were experiments trying to do the same thing with small calibre weapons using “flechettes” (google Steyr ACR) but that ultimately went nowhere.