[archy]

What's the advantage to breech loading mortars other than more control when the weapon is fired, i.e. the weapon is already loaded, waiting to fire?

Longer tubes are possible, offering better ballistic performance and longer range, since rounds don't have to be drop fired from the muzzle; longer barrels can also obviate the necessity for muzzle blast devices for crew hearing protection. Fire at lower angles of elevation is possible, including direct fire,[as with the improved US M19 60mm] making largebore mortars particularly useful in urban fighting/MOUT, and autoloaders are possible, as with the 4-shot Soviet 2B9 82-mm Vasilyek mortar found in both a towed/wheeled and MTLB tracked vehicle mounted version or can be mounted and transported in the cargo area of a modified GAZ-66 (4 x 4) 2000 kg truck.

There's also a similar 120mm MTLB tracked mortar carrier, not fitted with an autoloader- yet:

[Robert A Cook PE]

Short barrels require the crew lift the round "up" to the top of the barrel, and drop it down.

Real hard with a heavy round.

If you do this with a big barrel, you have to make a platform or stand or crane to lift it. Hard to be portable. Standing up high on a platform or using a crane exposes you to fire, if the enemy is close. Or then you dig down, but that takes time.

The real big coastal mortars (12" and 14") used against ships early in the century required crews of 12-16 men and permanent concrete "forts" ... They worked well against the Japanese at Corregidor in the nearby hills for a while, but enemy artillery eventually destroyed all the fixed mortars, the mounts, and killed many exposed troops trying to fire back.