81mm mortar dispersion

[Wreck]

Per a request over at the wiki, I tested out the fall of shot from German 81mm mortars.

The Test

All men (FOs, crews for onboard and offboard 81mm mortars, and HQ units) are regular/fit/0. My firing range put two mortars in a channel, so that combined they would have 44 shots; when shooting at the centerline of their channel both mortars would be at exactly the same range.

The testing method for onboard direct was very simple: aim at particular spot on the map. Fire until mortars are out of ammo. Take snapshot from above looking straight down, centered on targeted spot. You can eyeball the resulting snapshot to see the dispersion of the mortar shells. The only variable is the range.

I also tested offboard since it was pretty easy.

Results

You can see the particular screenshots I made here:

http://wreck0.imgur.com/all/

In particular, compare the one for 314m:

http://imgur.com/cBZX4

... to the one at 322m:

http://imgur.com/nIqWv

Here is what offboard looks like:

http://imgur.com/x6dUc

Summary of Results

From 100m to 320m, onboard mortars (fired direct or indirect) have very tight shot patterns. You can see where their fearsome reputation comes from: 90% of the shells land on the targeted action spot. Contrary to my expectation, there appears to be no increase in dispersion due to range, as the range varies from 100 to 320m.

At 320m, very suddenly the dispersion of shot for 81mm mortars appears to about triple. Most shots fall within the targeted action spot, or its immediate neighbors. I only tested up to 700m, and found no other breakpoints in performance. As with the shorter ranges, all ranges from 320-700m seem to have the same dispersion, although it is harder to tell in this case.

Offboard mortars fired at any range have the same dispersion, which is even larger than that seen for onboard mortars at 320+.

[akd]

Dispersion will continue to increase with range for on board assets. Try shooting at 2000m meters and this will become very apparent. You are correct that dispersion for offboard assets does not change as they have no variable range. At some particular range (probably between 500m-1000m) the dispersion for an onboard and offboard asset will be roughly the same (although pattern will vary).

The stepping you observed is interesting. Had not noticed this before. Some would be expected as propellant charges were added requiring a significant change in trajectory, but you would also expect some gradual increases with increasing flight time.

[Wreck]

Dispersion will continue to increase with range for on board assets. ... At some particular range (probably between 500m-1000m) the dispersion for an onboard and offboard asset will be roughly the same (although pattern will vary).

More testing shows the second stepping happens between 714 and 718 meters. Like the stepping at ~320m, it is very noticeable and abrupt.

The stepping you observed is interesting. Had not noticed this before. Some would be expected as propellant charges were added requiring a significant change in trajectory, but you would also expect some gradual increases with increasing flight time.

I also had considered the idea that CM is modelling more propellant, but this should increase uprange/downrange dispersion, not angular. Or at least not very much. The tube is still pointed the same way. Anyway, that's what my intuition says.

[Apa]

The flight time of a mortar shell _decreases_ as you shoot at longer distances. That is, until you change propellant amount.

This is because when you shoot as close as possible, you use the least propellant, and you shoot at an angle of close to 90 degrees. Hence your shot will go very high, and get more weather effects etc. When you shoot at longer distances, you shoot at lesser angle, all the way to (approximately) 45 degrees. At that angle your shot will go the farthest, and with least flight time.

Now if you need to shoot still further, you add propellant. And change your angle to somewhere around 70-80 degrees. Thus more height, more flight time and more dispersion. I bet this is what happens at around 320m.

[akd]

True dat...but lower trajectories also exaggerate range errors, no?

[Apa]

I should know (I was in a mortar team during my army time), but I am not sure about the lower trajectory causing more range error.

The reason is, no matter what your trajectory is, the shell is going to go down in steep angle. And because it goes down in steep angle, there isn't that much more range error. In any case not anything like what happens to artillery going down in small angles. There a small error in the starting angle could cause really big errors at the target.

I once got to lead fire. My experience (shooting with 120mm mortars) was that the error in range was less than 10m at the range of around 1000m, and there was virtually no angular error. On the other hand, spotting the right distance is _really_ hard.

We also did some "direct fire" shooting with 81mm mortars. At short ranges the accuracy is mind boggling. On a clear day you can see the shell the whole time. And at ranges of 200-400 meters, the pattern is maybe a meter wide (although it is somewhat hard to see the range dispersion).

My feeling about CMBN mortars is that everything is pretty well modeled, except the time it takes to FFE when the spotter is in direct visual or voice contact with the mortar team. If the spotter is regular+, the mortar team also, and the mortar team is already deployed, no suppression etc, the time to FFE should be at least half of what it is now. Another problem is infantry bunching up unrealistically close, which leads to maybe unrealistically high casualties.

As far as I understand there hasn't been any really dramatic differences in the technology used now vs the technology we had to learn in army. We knew how to operate with nothing more than a glorified compass, 50m set of wire, map and a stereoscopic range finder. Laser range finder, firing computer + GPS were also learned.

[YankeeDog]

True dat...but lower trajectories also exaggerate range errors, no?

Lower trajectories can increase range deviation though, as Apa notes, usually not that much in the case of weapons that fire at over 45 degrees. This is more of an issue with large guns firing at substantially below 45 degrees elevation. The 14" guns of the USS Texas were only capable of a max elevation of 22 degrees, so they were only capable of very shallow trajectory fire and should show substantial range deviation (which they don't in-game right now).

Assuming same charge setting, as noted, low trajectory also means a shorter flight time, which in turn means that the projectile is less susceptible to wind and other atmospheric variables. So especially in windy conditions, a lower trajectory can actually improve accuracy (in range and/or bearing, depending on the direction the wind is blowing).

[JasonC]

As a small dose of reality for these absurb figures, I would point out that new high tech *GPS guided* mortar shells, a huge advance over all existing lower tech items, are highly desired in the field because they get the CEP of mortar fire down to 10 meters. Meaning half of the shell will land that close and half will not. If, you know, the falling shell is talking to the global positioning system in real time to make tiny corrections to its trailing vanes to steer itself onto the target coordinates.

Meanwhile the CEP of unguided rounds at long range runs more like 80 to 140 meters.

Short range fire (a couple of km) with 81s, they can get the CEP down to about the round's blast radius - which for the CZ vs an upright target is around 35 meters. That makes 81mm mortars an effective weapon. A good crew can put a round within about half that distance within 3-5 rounds, with corrections, making it high effective in combat, because it can home in that much in a minute or two. "Home in" to landing 15 to 20 meters away, that is - which is enough to put anyone there, heads down.

These visions of 4 meter accuracy for 90% of the shells, on the other hand, you literally cannot get with a laser target designator and a homing, precision guided munition. It's just nonsense.

[akd]

Sorry Jason, but the fact that a PGM can achieve 10m CEP under all conditions does not mean that an unguided munition can never achieve the same probable error under any condition. Doesn't mean the current accuracy is right or wrong, but we can't get to the answer that way.

And would you really need a PGM direct laying a mortar on a target at 300m (setting spotting rounds aside)?

[gunnersman]

I recall that in CMx1 series all arty rounds created a beaten zone with more of an eliptical pattern that ran along the axis of the flight path of the projectile as opposed to the circular pattern in the pictures above. This struck me as appropriate since I think it more likely that rounds would fall short or long of the intended target as opposed to wide left or right.

[Wreck]

The question is thus raised. What was the CEP of WWII mortars? Was it, as we currently see, roughly 4m or so at ranges up to 320m?

I googled a bit for more info, but so far nothing too much. I did find a page at army.mil with this: "Current CEP for 120 mm mortars at their maximum range is 136 meters." To my knowledge the modern 120mm is based on the German WWII one; so I added a module of 120mm to my test and did a setup-plotted stonk. 22 shells later, I can see that offboard 120mm in CMBN has a CEP of 14m. A second stonk with more shells (32 + 2 spotting rounds) shows lower CEP of 12m.

This second stonk was done 1.4km from the observer; thus we can assume it must be at least say 2km from the mortars. If CEP is linear in distance from gun to target, then that would suggest a CEP of about 42m for the German 120mm at maximum range.

BTW in case people tried to look at my images and failed, I didn't have them public. Now it should work.

[akd]

The question is thus raised. What was the CEP of WWII mortars? Was it, as we currently see, roughly 4m or so at ranges up to 320m?

Closest I can get to an answer:

(6) The 60-mm mortar section can fire FPF that are about 70 meters wide and 35 meters deep. FPF are normally fired using impact or near-surface burst fuze settings. The mortar section has a single FPF assigned. Because of the light weight and small bursting radius of the mortar round, single mortar FPF have limited effectiveness. This does not preclude assigning of separate priority targets to each mortar squad. It means that such targets must be important enough to the commander to outweigh the need for FPF. The mortar section's FPF should be carefully integrated into the defensive fire plan of the company. It is most effective when the mortar section's FPF supplement the FPF of the battalion mortar platoon and the supporting field artillery. The accuracy and short minimum range of the M224 mean that the FPF can be close to friendly positions. (The M224 firing charge 0 has a maximum range probable error of only 3 meters.) Mortar FPF are always within small-arms range of friendly positions.

from FM 7-90

I googled a bit for more info, but so far nothing to much. I did find a page at army.mil with this: "Current CEP for 120 mm mortars at their maximum range is 136 meters."

To my knowledge the modern 120mm is based on the German WWII one; so I added a module of 120mm to my test and did a setup-plotted stonk. 22 shells later, I can see that offboard 120mm in CMBN has a CEP of 14m. A second stonk with more shells (32 + 2 spotting rounds) shows lower CEP of 12m.

This second stonk was done 1.4km from the observer; thus we can assume it must be at least say 2km from the mortars.

This cannot be assumed. Offboard assets have no variable distance from target.

If CEP is linear in distance from gun to target, then that would suggest a CEP of about 42m for the German 120mm at maximum range.

Not sure if you can get there that way.

[Wreck]

Offboard assets have no variable distance from target.

I tried a maximum size map (4km is evidently the max), firing 120mm at 3.8 km from the FO. CEP is 16m. So you seem to be right -- offboard has uniform CEP at all ranges. 16m is a dramatic difference from the 136m that the army gets from modern mortars.

[akd]

Except that is CEP at the absolute maximum range of the system. Differences in CEP at minimum and maximum range would be dramatic in and of themselves.

Also, although I have no particular reason to question the figure, I find it strange that I can only find it referred to in promotion or news coverage of the new PGM 120mm rounds.

[JasonC]

akd - and the fantasy continues...

For another dose of reality, read army field manual 6-30 on observed fire, aka the fire homing procedure for all indirect fire missions.

Unless otherwise specified, the target is assumed to be a circular area with a *radius* of 100 meters. Hmm, slightly larger than a CMx2 "action spot".

Fires within 600 meters of friendlies by artillery or mortars must be preceeded by the phrase "danger close" to the battery, to ensure they know that there are limits on sizes of adjustments and the like.

The normal deflection increment to be used in lateral adjustments is 5 mils. Typical examples of misses in the text are 25 or 35 mils off target. Notice, to get the miss distance in meters you have to multiply those mil figures by the range in kilometers. The manual specifies rounding these to the nearest 1000 meters because trying to get more refined than that about the range for these purposes is counterproductive.

When homing the rounds in range, successive bracketing is to proceed in increments 800, 400, 200, 100 meters. Obviously, dropping the higher parts of that series if the initial round is good enough - but getting an over and an under. Hasty bracketing means as soon as one over and under have been obtained, a final correction is called with FFE, otherwise one wants a 100 meter last adjust.

The manual is explicit that once the aim point is within 50 meters of the target, FFE must be called. There is no point in chasing aim points inside that radius because it is under the CEP of the guns, and the sheaf is aiming at a 100 meter radius circle. Corrections under 30 meters are not to be called in to the FDC.

These are only the actual procedures used to call for fire. They are based on the real world accuracy of unguided shells.

When registering, not firing on a target that may move where speed is a factor, tube artillery will continue adjustments down to 25 meters from the target, with adjustments called in minimum increments of 10 meters. Then the registration is completed. The next section details registrations for mortars and explicitly says the procedure is identical except that registration stops once rounds are within 50 meters from the target, and the minimum increment sent to the FDC is 25 meters. This reflects the manual's understanding that the intrinsic accuracy of mortars is *lower*, not higher, than the instrinsic accuracy of tube artillery. This should not be surprising - the muzzle velocity of mortars is around 200 meters per second as against 450 meters per second at the lowest for tube artillery, the mortar tubes are smoothbore (the 4.2 inch excepted), and the round fins rather than rifling-spin stablized. The only thing helping their average accuracy against these factors is their generally shorter range to their targets - and it merely mostly makes up for those limitations, it does not exceed them in importance.

Mortars are area fire weapons that utilize rate of fire and bursting radius to make up for their low inherent accuracy. When registered, a battery of 4 to 6 weapons firing 6 to 10 rounds in a single minute, each, with a bursting radius around 25-30 meters for an 81mm, can readily blanket their wide CEP. One or two dozen of the shells fired will land within a 50 meter circle around the aim point, and this will ensure that the aim point itself, and pretty much everything within 50 yards of that point, is blanketed by fragments dangerous to exposed infantry. Everything within 100 meters of the aim point risks fragment casualties, in fact, if not under cover.

60mm mortars are decidedly less effective, because their inherent accuracy is no higher, but their bursting charge is much lower. This gives a radius of danger for each shell of only about 15 meters, and the level of danger in the outer part of that radius is much smaller than for an 81mm round. Notice, even the raw radius given means the area covered by each round is only 18% of that covered by one round of 81mm. Fired in singles or groups of 3, they need to expend half their typical ammo load in the field to cover their beaten zone once. Can they be used to create mere danger and convince the enemy to go heads down while they are firing? Sure, and that is about the only thing that keeps them from being completely useless as a serious infantry weapon. They are nothing like the serious weapon system that an 81mm mortar is.

Instead in CM the 60mm mortars hit more frequently than the sniper rifles, and this is just plain nuts.

[Sgt Schultz]

I would tend to agree that all on-map mortars are too accurate at anything over 200-300 meters in direct-fire mode.

Off-maps are also a tad tight on their patterns, but less so.

BFC is usually fairly accurate in their simulation of weapon performance, so maybe there is evidence that they are working from that we have missed.

It could also be that real-world CEPs would overwhelm most Small or Tiny maps, which is where the "scale of the game" has its supposed sweet spot.

---

[Wreck]

I suspect that BFC are using firing range data. And also the sort of verbal testimony about firing ranges that inevitably come up in these threads: on the firing range, you can supposedly drop a mortar shell in a barrel. We find very different performance implied.

Although I think off-map indirect fire is probably a bit too accurate, I don't find it terribly unrealistic. What I do find unrealistic is the "sniper mortar". Direct fired mortars are deadly in CMBN, I think less based on their accuracy, than the fact that they are supercharged by the semi-borg sighting model. Regular infantry can do the same trick (to fire on areas where they see nothing themselves, not even a '?'-contact), but such fire rarely does that much. Also it is much more reasonable for a squad to expend 1 turn of fire on "nothing", when it has ammo to fire for 30 turns or whatever. Light mortars firing on areas they have no reason to fire at are much more problematic, because they chuck HE, with area effect. And also, that they will be expending 1/6 to 1/4 of their ammo load on "nothing".

The problem could be addressed simply, by disallowing area fire by mortars. I recognize several problems with the proposal, but it would be less of a distortion of reality than sniper mortars.

[SlowMotion]

Although I think off-map indirect fire is probably a bit too accurate, I don't find it terribly unrealistic. What I do find unrealistic is the "sniper mortar".

...

The problem could be addressed simply, by disallowing area fire by mortars. I recognize several problems with the proposal, but it would be less of a distortion of reality than sniper mortars.

If you read JasonC's post above, it says "Mortars are area fire weapons ". If mortars are too accurate and used like sniper rifles, I don't think the way to fix this is by banning the real use mode.

In JasonC's post it says that Fire For Effect must be called when spotting rounds are within 50 meters of your target. So how close exactly they are, 5m or 50m, that would be random. And *that* would mean mortars are no longer a sniper weapon.

[c3k]

JasonC,

Thanks for relaying the information from FM 6-30. When was that manual published? (Pre-WWII, during WWII, post-WWII) And, from where did you acquire it?

Thanks,

Ken

[TrailApe]

This struck me as appropriate since I think it more likely that rounds would fall short or long of the intended target as opposed to wide left or right

That would be correct if the gun batteries were directly behind you – but often what happens is that the Observer is off to one side of the line of gun to target. The unfortunate observer then gets the effect of Probable Error in Range (PeR) as a right/left correction, which it definetly is not.

Nightmare!

Woe betide the FOO who forgets the effect of zone and try’s to adjust it. At the Gun End, as the adjusting gun goes through its fifth apparently minor adjustment, conversation will start up in the CP and gunlines (all chafing to get a FFE order and get in on the action) on the topic of “Chasing Rabbits”.

As far as I understand there hasn't been any really dramatic differences in the technology used now vs the technology we had to learn in army. We knew how to operate with nothing more than a glorified compass, 50m set of wire, map and a stereoscopic range finder. Laser range finder, firing computer + GPS were also learned.

Not too far off the truth. Yes there is some mega whizzy kit out at the minute, but if you are operating for a few days away from your wheels/tracks and the opportunity to recharge power devices, the type of kit an OP party will carry is not that different from their forbearers in WW2. The best bit nowadays would be the GPS, it’s light and can run on easily portable power cells, however anything more space age (Thermal Imagers, Battlefield Radar and Laser Target Markers) and the amount of batteries required to power them for any reasonable amount time becomes prohibitive. ‘Steam Gunnery’ (Map, Bino’s, Compass and GPS) becomes the order of the day.

Caveat being GPS was a novelty just coming in as I was going out, so power packs might be a lot lighter nowadays.

[Wreck]

c3k: FM 6-30 can be found online easily. Here is a pdf on the Marine's site.

Here is a quote from page 1-5:

The traditional forward observer, equipped with a map, compass, and binoculars, can expect a mean target location error of about 500 meters. This is not enough for reliable first-round FFE or target suppression; it is no better than it was in World War II.

More googling shows that a better source would be FM 6-40, which was published in 1939. (6-30 was published in 1991.) Here is a pdf of FM 6-40.

[Wreck]

If you read JasonC's post above, it says "Mortars are area fire weapons ". If mortars are too accurate and used like sniper rifles, I don't think the way to fix this is by banning the real use mode.

I meant the "area fire" order in CMBN -- that is, direct fire on an action spot. The artillery interface is always area fire in a sense, and that I do not propose to change, beyond making it much faster to call mortars when you have a voice link. (They should have no more than a 1 minute delay.)

[Steiner14]

Wreck & JasonC,

great posts.

I don't understand why someone can defend the ridiculously über-weapon status mortars are right now, although the numbers speak for themselves.

I mean a factor of 3-5 less deviation than GPS guided shells? :eek: How stupid would be an army to invest into such ammo, if the cheap WWII technology would deliver unbeatable results against any stationary soft target?

IMO mortars destroy the balance, by rendering ATGs and HMGs almost useless because of their accuracy.

And it's also not fun to play, too: just move a mortar unit into place to take out every ATG or HMG is not only unrealistically, IMO it is also is BORING like hell.

BFC please fix or do somefink!

[Holien]

It certainly looks like it needs tweaking and the game play will change but I am not so sure I have had too many boring games despite this....

(Except maybe Chance Encounter which is now rated my worst scenario for H2H play and I don't think it would play too well as American Attacking either....)

A great thread and this and the Tank Firing accuracy is why I keep reading these posts to learn.....

[akd]

akd - and the fantasy continues...

For another dose of reality, read army field manual 6-30 on observed fire, aka the fire homing procedure for all indirect fire missions.

Unless otherwise specified, the target is assumed to be a circular area with a *radius* of 100 meters. Hmm, slightly larger than a CMx2 "action spot".

Fires within 600 meters of friendlies by artillery or mortars must be preceeded by the phrase "danger close" to the battery, to ensure they know that there are limits on sizes of adjustments and the like.

The normal deflection increment to be used in lateral adjustments is 5 mils. Typical examples of misses in the text are 25 or 35 mils off target. Notice, to get the miss distance in meters you have to multiply those mil figures by the range in kilometers. The manual specifies rounding these to the nearest 1000 meters because trying to get more refined than that about the range for these purposes is counterproductive.

When homing the rounds in range, successive bracketing is to proceed in increments 800, 400, 200, 100 meters. Obviously, dropping the higher parts of that series if the initial round is good enough - but getting an over and an under. Hasty bracketing means as soon as one over and under have been obtained, a final correction is called with FFE, otherwise one wants a 100 meter last adjust.

The manual is explicit that once the aim point is within 50 meters of the target, FFE must be called. There is no point in chasing aim points inside that radius because it is under the CEP of the guns, and the sheaf is aiming at a 100 meter radius circle. Corrections under 30 meters are not to be called in to the FDC.

These are only the actual procedures used to call for fire. They are based on the real world accuracy of unguided shells.

When registering, not firing on a target that may move where speed is a factor, tube artillery will continue adjustments down to 25 meters from the target, with adjustments called in minimum increments of 10 meters. Then the registration is completed. The next section details registrations for mortars and explicitly says the procedure is identical except that registration stops once rounds are within 50 meters from the target, and the minimum increment sent to the FDC is 25 meters. This reflects the manual's understanding that the intrinsic accuracy of mortars is *lower*, not higher, than the instrinsic accuracy of tube artillery. This should not be surprising - the muzzle velocity of mortars is around 200 meters per second as against 450 meters per second at the lowest for tube artillery, the mortar tubes are smoothbore (the 4.2 inch excepted), and the round fins rather than rifling-spin stablized. The only thing helping their average accuracy against these factors is their generally shorter range to their targets - and it merely mostly makes up for those limitations, it does not exceed them in importance.

Mortars are area fire weapons that utilize rate of fire and bursting radius to make up for their low inherent accuracy. When registered, a battery of 4 to 6 weapons firing 6 to 10 rounds in a single minute, each, with a bursting radius around 25-30 meters for an 81mm, can readily blanket their wide CEP. One or two dozen of the shells fired will land within a 50 meter circle around the aim point, and this will ensure that the aim point itself, and pretty much everything within 50 yards of that point, is blanketed by fragments dangerous to exposed infantry. Everything within 100 meters of the aim point risks fragment casualties, in fact, if not under cover.

60mm mortars are decidedly less effective, because their inherent accuracy is no higher, but their bursting charge is much lower. This gives a radius of danger for each shell of only about 15 meters, and the level of danger in the outer part of that radius is much smaller than for an 81mm round. Notice, even the raw radius given means the area covered by each round is only 18% of that covered by one round of 81mm. Fired in singles or groups of 3, they need to expend half their typical ammo load in the field to cover their beaten zone once. Can they be used to create mere danger and convince the enemy to go heads down while they are firing? Sure, and that is about the only thing that keeps them from being completely useless as a serious infantry weapon. They are nothing like the serious weapon system that an 81mm mortar is.

You cannot use these procedures to derive probable error for any particular weapon system at a particular range, as there are factors other than precision that play a role. An example of an actual figure given for probable error (range) of a weapon system in a particular range band has already been posted to this thread: 3m for the M224 firing charge 0. That is as precise, or more precise, than field artillery, but only in a narrow range band.

Should off-map mortars be given a greater default range from target? Perhaps.