The 1st cannons, pistols and rifles all had smooth bores.

Then someone noticed that putting spiraled grooves down
the inside of a barrel made the weapons more accurate.

This is because smooth bores do not impart
a spin on the projectiles they fire.

When an object traveling through the air has no spin,
it creates excessive turbulence.

This turbulence disturbs the course of the projectile as it speeds to it’s target*.

However, the grooves in rifled barrels 'grab’ a projectile
as it travels through the gun barrel, and impart a spin on it.

Can you see the grooves?

This spin breaks up the turbulence, allowing the projectile to fly straight.

For this reason virtually every gun
(except shotguns that scatter shrapnel)
made in the 20th century has been rifled.

Rifle twist viewed from a cut-away. - Mossberg

That was until the development of
shaped-charge anti-tank rockets in W.W.II.

This type of round must focus it’s explosive charge
on a very small point when it hit’s the target.

Centrifugal force, created by the spin of rifled barrels, scatters
the shaped-charge projectile’s explosion, making it ineffective.

This is why the M1A1 uses a smooth bore.

In order to break up the turbulence w/ out spinning,
the shaped charge projectile use fins like those
found on a dart used in many an English pub.

Mortars also use a smooth bore w/ fins on the projectile.

This is because the projectile must be
dropped down the muzzle of the tube.
There must be a space between the projectile and muzzle
so it can reach sufficient speed to detonate
the propellant in it's base.

Rifle grooves must come in contact w/ the projectile which would slow it's fall.

Many muzzle-loading rifles also used smooth bores for the same reason.

*In the sport of baseball this effect is used to throw a 'knuckle ball’.
The ball is held w/ either the fingernails dug in to the ball.
When the pitcher releases the ball he flicks his fingers outward.
This overcomes the natural spin produces under normal conditions.
If it done right (very few who try ever master it) the ball
will spin less than one time, as it travel to the batter.
This lack of spin creates turbulence that can make to ball
curve as much as 10 ft., as it gets to the batter.
Some have been known to change their curve angle
5 times before they get to hone plate.