Their uses, and usefulness against certain formations.
Other than heavy artillery, the mortar gives an imperial guard commander the only other readily available option for indirect fire. This heavy weapon can take two forms: the individual mortars carried by infantry squads, and attendants to an officer, and the three mortar heavy support squad. First, let us look at the effectiveness of the mortar.
Note that, as the mortar is a blast weapon, and one which has a tendency to scatter, rather than be a simple matter of hitting and missing, I will use the following terms. The first is a "hit" whereby the mortar lands more or less exactly where it was supposed to. A "near miss" is when the mortar scatters, but not so far that it can't at least wing an infantry man or two. A "far miss" is when the mortar scatters so badly, that no targets in the intended unit are hit.
Let us also assume, that over three phases of a battle, that any one mortar will get a result of one of each of these three categories. Thus, over three turns, we can roughly expect the following result:
Heavy Bolter: 4.5 hits, 3.78 kills (1.9 if the unit is in good cover)
Missile Launcher (frag): 1.5 hits (let us say about 1 hit and 6 partials) for 1.7 kills (1.3 if in good cover)
Mortar: (let us assume a similar 1 hit and 6 partials) for 1.7 kills (1.3 if in good cover)
Heavy Bolter: 4.5 hits, .99 kills (irrespective of cover).
Missile launcher (krak): 1.5 hits, 1.26 kills*
Mortar: 1 hit and 6 partials, for .66 kills (irrespective of cover)
*Note that the # of kills of a krak missile drops down to .63 if the unit fired against is in cover. If a frag missile is used, then the weapon inflicts .66 kills, irrespective of cover.
It is important to note as well, that, because fragmentation missiles and mortars use a blast radius to inflict damage, their kill quotient varies as described below. Note, for example, that if a mortar hit dead on a 10-man group of recently deepstruck space marines, it is not unthinkable for a single mortar hit to kill twice as many in one turn than a heavy bolter could in three.
As well, it should be noted that light, open topped vehicles are particularly vulnerable to mortar fire. If a mortar hits such a vehicle, it has slightly better chance than to destroy said vehicle once every ten hits, about half the takedown power of a heavy bolter (which, though not ideal, is not completely useuess).
MORTARS AND STRATEGY
As said before, the mortar is unique to other Imperial Guard heavy weapons in that it can shoot at that which it cannot see (without being seen in return, of course). Thus, mortars can play an integral part in forcing an enemy formation to attack your lines (to their impending peril). As well, this allows a commander to be able attack units which an opposing commander desires to keep in safety by keeping them out of line of sight.
Indeed, other than a strategic tool, mortars have a hidden firepower bonus. Namely, a mortar is guaranteed to be able to shoot at enemy units every turn that it's crew survives. An enemy can not move their forces in such a way where they become immune to the weapon's firepower. Thus, while a heavy bolter may generally outperform a mortar when it has line of sight, it loses out significantly when the enemy refuses to present a proper target for the heavy bolter. Thus, the mortar becomes the ideal weapon for several applications as, despite it's low accuracy, strength, and armor penetrability, it may well be the only heavy weapon that can even shoot at some types of units in the first place.
While a mortar is good for ensuring that opposing armies whose strength relies on static firepower in strong positions abandon their position to attack your army (otherwise, they would be left with no recourse to your shelling, so long as you are able to prevent displaying any targets to the opposing force), but as well, they can keep onrushing opponents at bay. This is, of course, because of the mortar's ability to confuse enemy units, and cause them to be pinned to the ground. Certainly, pinning enemy units is far from certain, but it does, however, cause grand assaults to become more piecemeal, and even one unit being tardy can alter the outcome of the assault on your positions. As well, if you have many mortars on the field, your force has a much greater chance of causing pinning to the opposing force, to the point where it almost becomes reliable in sufficient numbers.
As well, though the weapon has obvious strategic value to trading off accuracy for the ability to fire indirectly, there is nothing stopping an imperial commander from firing mortars at targets that they are already able to see. In this case, the accuracy of the mortar is greatly improved. In this case, assuming that the units are spaced out as much as they were on the above study of effectiveness, a single mortar is likely to put out 2 hits and 9 partials in three turns, bringing the kill count against light infantry in cover up to 2.1 and 1.1 against heavy infantry (compared to the 1.9 and .99 of the heavy bolter, respectively).
Finally, as was also noted before, and will be studied shortly, the mortar's effectiveness is based in part on the displacement of enemy troops. That being said, there are several times when enemy troops are likely to be in very close quarters, including deepstriking, getting out of a transport (voluntarily or otherwise), or when trying to fit an entire unit into cover. As well, if you are able to force your enemy to cluster together (through creating powerful firing lanes, for example), a skilled commander can MAKE their mortars more effective by forcing their opponent into closer formations.
DISPLACEMENT AND EFFECTIVENESS
In this section, we will look into the effects of the displacement of enemy troops and how it affects both a single mortar, and a three mortar barrage.
The first situation that can be found is that of the enemy troops having almost no displacement, that is, they are tightly crowded together. This is most common for troops that are deepstriking, getting out of transports, or are trying to hide in a piece of terrain.
In this case, the mortar has hit its target dead on. Obviously, the effects of this are devastating, as 7 of the enemy soldiers are hit outright with a further 2 being partially enveloped in the blast.
This mortar, however, has taken a scatter of only 3". Note that while a dead on hit causes mass carnage, the tight formation makes it so that any scatter at all is likely to leave the unit unhurt. Thus, the opposing player has right to fear a direct hit, but has a greater deal of comfort knowing that none of his troops will likely be hit in the first place.
In this case, a triple mortar barrage has had its ranging shot hit dead on. Note, however, that subsequent shots can not land directly on the same place that the ranging shots. This means that a triple mortar barrage is not likely to do much more damage, if any, to tightly packed troops than a single mortar hitting.
Note, however, that a mortar barrage suffers far less from missing than an individual mortar. In this extreme case, the ranging shot fell afoul by 6". Remember that if a non-ranging shot hits, the blast may be placed touching ANY template already laid. This means that if even one non-ranging shotis a hit, or there are some scatters in favorable directions, it is likely that a blast will at least partially envelop the enemy formation.
So, we can draw a few conclusions from this. The main conclusion is that against tightly packed troops, it is often better to take multiple single mortars than a single, multiple mortar barrage. This is because a single mortar, if it hits, does the same amount of damage as a barrage, and, even though the triple mortar barrage is far more likely to hit with at least one blast marker with it's salvo, statistically, with three individual mortars, one is likely to roll at least one hit.
So, while there is the potential for much greater damage (imagine three individual mortars all hitting), there is the chance that all three single mortars will scatter, which, given such tight clustering, will mean little damage. In short, three mortars gives you more potential carnage against bunched up troops, while a three mortar barrage gives you more security that you will at least do some damage.
The second situation to look at is if your enemy's troops are in a somewhat spread out formation.
In this case, the mortar has hit dead on, causing a solid hit, along with a lot of partials (in this case, 4). No matter the exact position of the troops, there is likely to be a lot of partial hits.
In this case, the mortar has scattered 4" away. Note that the mortar still has problems getting any hits in when it scatters. It should be noted, though, that if it had scattered 3" as in the previous example, it would have picked up a couple of partials. This is, in the end, the best that you can expect from a missed mortar with this level of displacement.
In this case, the mortar barrage does very well. While it is still not likely to do much better than one or two hits, it is likely to cover most of the enemy troops at least once, causing a large amount of partial hits (in this case, 9)
In this case, the ranging mortar missed in the same way, but with a single hit or favorable direction for the scatter, you can see how the barrage could easily arc back and hit several members of the opposing squad
At this level of displacement, having three mortars and having a three mortar team begin to balance out. Here, three direct hits with three separate mortars would only do slightly better than a direct hit with the ranging shot on the mortars, while the barrage's ability to "walk" it's shots towards the enemy makes it output more damage than the single mortar missing.
Assuming that the three mortars scored a hit, a near miss, and a far miss, the end result would likely be something on the order of 1 hit and 6 partials, something which the mortar barrage can easily replicate, even if the ranging shot misses.
Now, let us consider the final form of displacement. Let us assume that the enemy commander is wise to your plan of using mortars and has spaced his troops far apart from one another so as to lessen the damage from the blasts.
As it would be expected, if the shot from a single mortar hits, it will only hit one member of the squad. That being said, even if the shot takes a very short scatter in a good direction, the shot is unlikely to pick up more than a couple of partial hits.
For the first time thus far, scattering does not doom the single mortar. In this case, a moderate scatter still yields a solid hit, while it is possible to glean a few partials even from a far scatter
As we can see once again, a triple mortar barrage has the ability to hit a large portion of the squad with a successful hit, even though the squad is displaced. It should also be noted that a hit, or favorable scatter direction has the ability to get at least one good hit, or a fair number of partials.
In this case, the ranging shot scattered a full 6" away. Note that this does not reduce the number of hits by a substantial amount.
In this set of circumstances, the three mortar barrage and three individual mortars are almost equal. This is because any amount of hitting or scattering is likely to produce roughly the same results. The three mortar barrage does carry a slight advantage, though, in that you will often be able to control the direction of one of the non ranging shots scatters, allowing you to make the best out of an otherwise bad situation.
It should be noted that there is a good deal of reliability for both types of mortar configuration. Even with great accuracy, or terrible accuracy, after the partials are resolved, it is unlikely that more than two or three members of the opposing squad will be hit. On the plus side, it is almost impossible for mortars to have any result other than this, on the minus, this is a less than desirable result, no matter how consistent.
The first thing to note is that displacement of ones forces does not do a tremendous amount to reduce the effectiveness of mortars against them. Yes, the potential for raw number of hits goes down, but at the same time, it makes an otherwise inaccurate weapon more and more likely to hit. This means that the dispersed squads are more likely to make pinning checks due to more consistent hitting. As well, a commander who bunches his forces together, while risking a big hit, is also creating the opportunity for no mortars to ever hit: a risk that some might be willing to take.
Three individual mortars are good, as, against tight-knit squads, they have the potential to do a LOT of damage (try not to smile too hard when three mortars hit dead on for 21 hits and 6 partials on a 10-member squad). That being said, this requires three separate units to do nothing else other than sit and shell enemy forces, something which may be logistically undesirable.
A three mortar support squad is good, because, though it is less likely to do as much damage in perfect conditions, it is much more likely to do SOME damage in less than ideal situations, and it is quite a bit more likely to do some damage in poor conditions, especially against tightly packed formations. Their main drawback, however, is the ability to cause pinning checks on more than one unit per turn (or multipli pinning checks on a single unit), and their relative expense per mortar, which is slightly higher.
As well, note that the mortar becomes less effective the fewer members of a squad there are. Not only does it make scattering more likely to miss altogether, but it as well lowers the amount of hits that mortars can make. Thus, mortars are often best used to soften up enemy formations before hitting them with weapons which are not as dependent on squad size as the mortar.
As a final note, as well, remember that mortars are indiscriminate. Thus, it is possible for a ranging shot to scatter 6" off of the intended target, but to make a chain in the OTHER direction, hitting a unit over 13" away from the intended landing point, even if the unit hit was normally untargetable, or was otherwise not the intended target.
In conclusion, the mortar is a weapon which has a great deal of potential, even if it is difficult to actualize, and gives an Imperial commander options that no other weapon (save artillery) can give. When used wisely, few enemies of the Imperium are able to withstand a coordinated attack that is supported by a proper use of the mortar.