ORIGINAL: Bogo Mil
I didn't say they were completely unable to do indirect fire, but most units were very badly trained at it.
Pre-planned bombardement against well known enemy positions was possible - this is the "easiest" way of indirect fire. But in a moving battlefield, when the batteries have to calculate their aiming themselves and time is an issue, they were usually not able to do it fast and accurate enough. Thus they usually relied on direct fire during the first year of the Great Patriotic War.
The Warsaw Pact was still relying heavily on direct fire support in the 1970s and 1980s. In analyses, we generally carried WP DF artillery as having about 3 times the firepower of indirect artillery of the same calibre (ceteris paribus). There were two downsides: ammunition supply and flexibility.
In these analyses, we considered the difference between general support and direct support as well. On a scale of 1-20, a light direct-fire howitzer in direct support was about 10 in that mission, with the lighter the calibre the higher the firepower. Guns, howitzers, and mortars of the same calibre and at the same rate of fire were 1:2:3 in relative firepower based on lethal area, so a 75 mm gun in direct fire was about the same effectiveness as the light howitzer--the howitzer was better in indirect fire. The light howitzer in direct support indirect-fire was 3. Medium caliber howitzers were 2, and heavy howitzers were 1, mostly due to less lethal area per minute. On the other hand, for general support missions, where shell explosive charge was more important than lethal area (which was based on fragmentation), for example against fortifications, artillery emplacements, armour or dug-in positions, the corresponding ratings were 1 for a light weapon, 2 for a medium weapon, and 3 for a heavy weapon.






